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Abstract:

A method and apparatus for determining oversensing of cardiac signals that
includes a housing containing electronic circuitry, an electrode coupled
to the electronic circuitry to sense cardiac signals, and a processor,
positioned within the housing, to determine an oversensing characteristic
associated with the cardiac signals sensed over a predetermined sensing
window, and to identify oversensing in response to the determined
oversensing characteristic.

Claims:

1. A medical device for determining oversensing of cardiac signals,
comprising:a housing containing electronic circuitry;an electrode coupled
to the electronic circuitry to sense cardiac signals; anda processor,
positioned within the housing, to determine an oversensing characteristic
associated with the cardiac signals sensed during a predetermined sensing
window, and to identify oversensing in response to the determined
oversensing characteristic.

2. The device of claim 1, further comprising a control unit to control
transitioning of the device between a first state corresponding to a
cardiac event being detected and a second state corresponding to the
cardiac event not being detected in response to the identifying.

3. The device of claim 1, wherein the cardiac signals are sensed from a
non-intravenous location.

4. The device of 1, wherein determining an oversensing characteristic
includes determining a first oversensing characteristic and a second
oversensing characteristic, the first oversensing characteristic
corresponding to a morphology of the sensed cardiac signals sensed during
a predetermined sensing window, and the second oversensing characteristic
corresponding to RR-intervals associated with the sensed cardiac signals.

5. A method of determining oversensing in a medical device,
comprising:sensing cardiac signals;determining an oversensing
characteristic associated with the cardiac signals sensed during a
predetermined sensing window; andidentifying oversensing in response to
the determined oversensing characteristic.

6. The method of claim 5, further comprising transitioning between a first
state corresponding to a cardiac event being detected and a second state
corresponding to the cardiac event not being detected in response to the
identifying.

7. The method of claim 5, wherein the cardiac signals are sensed from a
non-intravenous location.

8. The method of 5 wherein determining an oversensing characteristic
includes determining a first oversensing characteristic and a second
oversensing characteristic, the first oversensing characteristic
corresponding to a morphology of the sensed cardiac signals sensed during
a predetermined sensing window, and the second oversensing characteristic
corresponding to RR-intervals associated with the sensed cardiac signals.

9. A method of determining oversensing in a medical device,
comprising:sensing cardiac signals from a plurality of electrodes, the
plurality of electrodes forming a first sensing vector and a second
sensing vector;determining whether the cardiac signals sensed along the
first sensing vector are reliable;determining whether the cardiac signals
sensed along the second sensing vector are reliable;determining an
oversensing characteristic associated with the cardiac signals sensed
during a predetermined sensing window; andidentifying oversensing in
response to the determined oversensing characteristic.

10. The method of claim 9, wherein in response to both the cardiac signals
sensed along the first sensing vector and along the second sensing vector
being reliable, determining an oversensing characteristic
comprises:determining a first spectral width corresponding to the cardiac
signals sensed along the first sensing vector and a second spectral width
corresponding to the cardiac signals sensed along the second sensing
vector;identifying one of the first spectral width and the second
spectral width as a preferred spectral width; andcomparing the preferred
spectral width to a spectral width threshold.

11. The method of claim 9, wherein in response to both the cardiac signals
sensed along the first sensing vector and along the second sensing vector
being reliable, determining an oversensing characteristic
comprises:determining a first metric of signal energy content
corresponding to the cardiac signals sensed along the first sensing
vector and a second metric of signal energy content corresponding to the
cardiac signals sensed along the second sensing vector;identifying one of
the first metric of signal energy content and the second metric of signal
energy content as a preferred metric of signal energy content;
andcomparing the preferred metric of signal energy content to a metric of
signal energy content threshold.

12. The method of claim 9, wherein in response to both the cardiac signals
sensed along the first sensing vector and along the second sensing vector
being reliable, determining an oversensing characteristic
comprises:determining a first heart rate metric difference corresponding
to the cardiac signals sensed along the first sensing vector and a second
heart rate metric difference corresponding to the cardiac signals sensed
along the second sensing vector;identifying one of the first heart rate
metric difference and the second heart rate metric difference as a
preferred heart rate metric difference; andcomparing the preferred heart
rate metric difference to a heart rate metric difference threshold.

13. The method of claim 9, wherein the plurality of electrodes are
positioned at a non-intravenous location.

14. The method of claim 9, wherein in response to both the cardiac signals
sensed along the first sensing vector and along the second sensing vector
being reliable, determining an oversensing characteristic
comprises:determining a first spectral width corresponding to the cardiac
signals sensed along the first sensing vector and a second spectral width
corresponding to the cardiac signals sensed along the second sensing
vector;identifying one of the first spectral width and the second
spectral width as a preferred spectral width;comparing the preferred
spectral width to a spectral width threshold;determining a first metric
of signal energy content corresponding to the cardiac signals sensed
along the first sensing vector and a second metric of signal energy
content corresponding to the cardiac signals sensed along the second
sensing vector;identifying one of the first metric of signal energy
content and the second metric of signal energy content as a preferred
metric of signal energy content;comparing the preferred metric of signal
energy content to a metric of signal energy content threshold;determining
a first heart rate metric difference corresponding to the cardiac signals
sensed along the first sensing vector and a second heart rate metric
difference corresponding to the cardiac signals sensed along the second
sensing vector;identifying one of the first heart rate metric difference
and the second heart rate metric difference as a preferred heart rate
metric difference; andcomparing the preferred heart rate metric
difference to a heart rate metric difference threshold, wherein
oversensing is identified in response to the comparings.

15. The method of claim 9, further comprising controlling the rate at
which the determining of the oversensing characteristic is performed.

16. The method of claim 9, wherein in response to one of the cardiac
signals sensed along the first sensing vector and the cardiac signals
sensed along the second sensing vector reliable being reliable,
determining an oversensing characteristic comprises:determining a
spectral width corresponding to the one of the cardiac signals sensed
along the first sensing vector and the cardiac signals sensed along the
second sensing vector; andcomparing the spectral width to a spectral
width threshold.

17. The method of claim 9, wherein in response to one of the cardiac
signals sensed along the first sensing vector and the cardiac signals
sensed along the second sensing vector being reliable, determining an
oversensing characteristic comprises:determining a metric of signal
energy content corresponding to the one of the cardiac signals sensed
along the first sensing vector and the cardiac signals sensed along the
second sensing vector; andcomparing the metric of signal energy content
to a metric of signal energy content threshold.

18. The method of claim 9, wherein in response to one of the cardiac
signals sensed along the first sensing vector and the cardiac signals
sensed along the second sensing vector being reliable, determining an
oversensing characteristic comprises:determining a heart rate metric
difference corresponding to the one of the cardiac signals sensed along
the first sensing vector and the cardiac signals sensed along the second
sensing vector; andcomparing the heart rate metric difference to a heart
rate metric difference threshold.

19. The method of claim 9, wherein, in response to one of the cardiac
signals sensed along the first sensing vector and the cardiac signals
sensed along the second sensing vector reliable being reliable,
determining an oversensing characteristic comprises:determining a
spectral width corresponding to the one of the cardiac signals sensed
along the first sensing vector and the cardiac signals sensed along the
second sensing vector;comparing the spectral width to a spectral width
threshold;determining a metric of signal energy content corresponding to
the one of the cardiac signals sensed along the first sensing vector and
the cardiac signals sensed along the second sensing vector;comparing the
metric of signal energy content to a metric of signal energy content
threshold;determining a heart rate metric difference corresponding to the
one of the cardiac signals sensed along the first sensing vector and the
cardiac signals sensed along the second sensing vector; andcomparing the
heart rate metric difference to a heart rate metric difference threshold,
wherein oversensing is identified in response to the comparings.

20. The method of claim 18, further comprising controlling the rate at
which the determining of the oversensing characteristic is performed,
wherein the controlling is set subsequent to the determining a heart rate
metric difference.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001]Cross-reference is hereby made to the commonly-assigned related U.S.
Applications, attorney docket number P0027429.01, entitled "METHOD AND
APPARATUS FOR DETECTING ARRHYTHMIAS IN A MEDICAL DEVICE", to Ghanem et
al.; P0027429.02, entitled "METHOD AND APPARATUS FOR DETECTING
ARRHYTHMIAS IN A MEDICAL DEVICE", to Ghanem et al.; and P0027429.03,
entitled "METHOD AND APPARATUS FOR DETECTING ARRHYTHMIAS IN A MEDICAL
DEVICE", to Ghanem et al., filed concurrently herewith and incorporated
herein by reference in their entireties.

FIELD OF THE INVENTION

[0002]The present invention generally relates to an implantable medical
device system, and more particularly to a method and apparatus for
detecting arrhythmias in a subcutaneous medical device.

BACKGROUND OF THE INVENTION

[0003]Many types of implantable medical devices (IMDs) have been implanted
that deliver relatively high-energy cardioversion and/or defibrillation
shocks to a patient's heart when a malignant tachyarrhythmia, e.g.,
ventricular tachycardia or ventricular fibrillation, is detected.
Cardioversion shocks are typically delivered in synchrony with a detected
R-wave when fibrillation detection criteria are met, whereas
defibrillation shocks are typically delivered when fibrillation criteria
are met and an R-wave cannot be discerned from the electrogram (EGM).

[0004]The current state of the art of ICDs or implantable
pacemaker/cardioverter/defibrillators (PCDs) includes a full featured set
of extensive programmable parameters which includes multiple arrhythmia
detection criteria, multiple therapy prescriptions (for example,
stimulation for pacing in the atrial, ventricular and/or both chambers,
bi-atrial and/or bi-ventricular pacing, arrhythmia overdrive or
entrainment stimulation, and high level stimulation for cardioversion
and/or defibrillation), extensive diagnostic capabilities and high speed
telemetry systems.

[0005]Current technology for the implantation of an IMD uses a transvenous
approach for cardiac electrodes and lead wires. The defibrillator
canister/housing is generally implanted as an active can for
defibrillation and electrodes positioned in the heart are used for
pacing, sensing and detection of arrhythmias.

[0006]Attempts are being made to identify patients who are asymptomatic by
conventional measures but are nevertheless at risk of a future sudden
death episode. Current studies of patient populations, e.g., the MADIT II
and SCDHeFT studies, are establishing that there are large numbers of
patients in any given population that are susceptible to sudden cardiac
death, that they can be identified with some degree of certainty and that
they are candidates for a prophylactic implantation of a defibrillator
(often called primary prevention).

[0007]One option proposed for this patient population is to implant a
prophylactic subcutaneous implantable device (SubQ device). As SubQ
device technology evolves, it may develop a clear and distinct advantage
over non-SubQ devices. For example, the SubQ device does not require
leads to be placed in the bloodstream. Accordingly, complications arising
from leads placed in the cardiovasculature environment are eliminated.
Further, endocardial lead placement is not possible with patients who
have a mechanical heart valve implant and is not generally recommended
for pediatric cardiac patients. For these and other reasons, a SubQ
device may be preferred over an ICD.

[0008]There are technical challenges associated with the operation of a
SubQ device. For example, SubQ device sensing is challenged by the
presence of muscle artifact, respiration and other physiological signal
sources. This is particularly because the SubQ device is limited to
far-field sensing since there are no intracardial or epicardial
electrodes in a subcutaneous system. Further, sensing of atrial
activation from subcutaneous electrodes is limited since the atria
represent a small muscle mass and the atrial signals are not sufficiently
detectable transthoracically.

[0009]Yet another challenge could occur in situations where it is
desirable to combine a SubQ device with an existing pacemaker (IPG) in a
patient. While this may be desirable in a case where an IPG patient may
need a defibrillator, a combination implant of a SubQ device and an IPG
may result in inappropriate therapy by the SubQ device, which may pace or
shock based on spikes from the IPG. Specifically, each time the IPG emits
a pacing stimulus, the SubQ device may interpret it as a genuine cardiac
beat. The result can be over-counting beats from the atrium, ventricles
or both; or, because of the larger pacing spikes, sensing of arrhythmic
signals (which are typically much smaller in amplitude) may be
compromised.

[0010]Therefore, for these and other reasons, a need exists for an
improved method and apparatus to reliably sense and detect arrhythmias in
a subcutaneous device, while rejecting noise and other physiologic
signals.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]Aspects and features of the present invention will be appreciated as
the same becomes better understood by reference to the following detailed
description of the embodiments of the invention when considered in
connection with the accompanying drawings, wherein:

[0012]FIG. 1 is a schematic diagram of an exemplary subcutaneous device in
which the present invention may be usefully practiced;

[0013]FIG. 3 is an exemplary schematic diagram of electronic circuitry
within a hermetically sealed housing of a subcutaneous device of the
present invention;

[0014]FIG. 4 is a schematic diagram of signal processing aspects of a
subcutaneous device according to an exemplary embodiment of the present
invention;

[0015]FIG. 5 is a state diagram of detection of arrhythmias in a
subcutaneous device according to an embodiment of the present invention;

[0016]FIG. 6 is a flow chart of a method for detecting arrhythmias in a
subcutaneous device according to an embodiment of the present invention;

[0017]FIGS. 7A-7I are flow charts of a method for detecting arrhythmias in
a subcutaneous device according to an embodiment of the present
invention;

[0018]FIG. 8 is a graphical representation of sensing of cardiac activity
according to an embodiment of the present invention;

[0019]FIG. 9A is a graphical representation of a determination of whether
a signal is corrupted by muscle noise according to an embodiment of the
present invention;

[0020]FIG. 9B is a flowchart of a method of determining whether a signal
is corrupted by muscle noise according to an embodiment of the present
invention;

[0021]FIG. 9C is a flowchart of a method of determining whether a signal
is corrupted by muscle noise according to an embodiment of the present
invention;

[0022]FIG. 10 is a graphical representation of a VF shock zone according
to an embodiment of the present invention;

[0023]FIGS. 11A and 11B are graphical representations of the determination
of whether an event is within a shock zone according to an embodiment of
the present invention;

[0024]FIG. 12 is a graphical representation of a shock zone according to
an embodiment of the present invention;

[0025]FIG. 13 is a graphical representation of the determination of
whether an event is within a shock zone according to an embodiment of the
present invention;

[0026]FIGS. 14A-14C are graphical representations illustrating the
occurrence of oversensing due to a slow monomorphic ventricular
tachycardia with a wide QRS complex;

[0027]FIG. 15 is a flowchart of a method for detecting cardiac events in a
medical device according to an embodiment of the present invention;

[0028]FIGS. 16A and 16B are flowcharts of a method of determining whether
oversensing has occurred according to an embodiment of the present
invention;

[0029]FIG. 17 is a flowchart of a method of determining whether
oversensing has occurred according to an embodiment of the present
invention;

[0030]FIG. 18 is a flowchart of a method of determining whether
oversensing has occurred according to an embodiment of the present
invention;

[0031]FIGS. 19A and 19B are graphical representations of determining a
corrected heart rate in response to oversensing according to an
embodiment of the present invention;

[0032]FIGS. 20A and 20B are flowcharts of a method of determining a
corrected heart rate in response to oversensing according to an
embodiment of the present invention;

[0033]FIG. 21 is a flowchart of a method for determining a corrected rate
according to an embodiment of the present invention;

[0034]FIG. 22 is a flowchart of a method of determining a corrected heart
rate in response to oversensing according to an embodiment of the present
invention;

[0035]FIG. 23 is an exemplary schematic diagram of a buffer of RR
intervals generated according to an embodiment of the present invention;

[0036]FIG. 24 is a flowchart of a method for determining a corrected rate
according to an embodiment of the present invention;

[0037]FIG. 25 is an exemplary schematic diagram of a buffer of RR
intervals generated according to an embodiment of the present invention;
and

[0038]FIG. 26 is a flowchart of a method for detecting cardiac events in a
medical device according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0039]FIG. 1 is a schematic diagram of an exemplary subcutaneous device in
which the present invention may be usefully practiced. As illustrated in
FIG. 1, a subcutaneous device 14 according to an embodiment of the
present invention is subcutaneously implanted outside the ribcage of a
patient 12, anterior to the cardiac notch. Further, a subcutaneous
sensing and cardioversion/defibrillation therapy delivery lead 18 in
electrical communication with subcutaneous device 14 is tunneled
subcutaneously into a location adjacent to a portion of a latissimus
dorsi muscle of patient 12. Specifically, lead 18 is tunneled
subcutaneously from the median implant pocket of the subcutaneous device
14 laterally and posterially to the patient's back to a location opposite
the heart such that the heart 16 is disposed between the subcutaneous
device 14 and the distal electrode coil 24 and distal sensing electrode
26 of lead 18.

[0040]It is understood that while the subcutaneous device 14 is shown
positioned through loose connective tissue between the skin and muscle
layer of the patient, the term "subcutaneous device" is intended to
include a device that can be positioned in the patient to be implanted
using any non-intravenous location of the patient, such as below the
muscle layer or within the thoracic cavity, for example.

[0041]Further referring to FIG. 1, a programmer 20 is shown in telemetric
communication with subcutaneous device 14 by an RF communication link 22.

[0042]Communication link 22 may be any appropriate RF link such as
Bluetooth, WiFi, MICS, or as described in U.S. Pat. No. 5,683,432
"Adaptive Performance-Optimizing Communication System for Communicating
with an Implantable Medical Device" to Goedeke, et al and incorporated
herein by reference in its entirety.

[0043]Subcutaneous device 14 includes a housing 15 that may be constructed
of stainless steel, titanium or ceramic as described in U.S. Pat. No.
4,180,078 "Lead Connector for a Body Implantable Stimulator" to Anderson
and U.S. Pat. No. 5,470,345 "Implantable Medical Device with
Multi-layered Ceramic Enclosure" to Hassler, et al, both incorporated
herein by reference in their entireties. The electronics circuitry of
SubQ ICD 14 may be incorporated on a polyimide flex circuit, printed
circuit board (PCB) or ceramic substrate with integrated circuits
packaged in leadless chip carriers and/or chip scale packaging (CSP).

[0044]Subcutaneous lead 18 includes a distal defibrillation coil electrode
24, a distal sensing electrode 26, an insulated flexible lead body and a
proximal connector pin 27 (shown in FIG. 2) for connection to the housing
15 of the subcutaneous device 14 via a connector 25. In addition, one or
more electrodes 28 (shown in FIG. 2) are positioned along the outer
surface of the housing to form a housing-based subcutaneous electrode
array (SEA). Distal sensing electrode 26 is sized appropriately to match
the sensing impedance of the housing-based subcutaneous electrode array.

[0045]It is understood that while device 14 is shown with electrodes 28
positioned on housing 15, according to an embodiment of the present
invention electrodes 28 may be alternatively positioned along one or more
separate leads connected to device 14 via connector 25.

[0046]Continuing with FIG. 2, electrodes 28 are welded into place on the
flattened periphery of the housing 15. In the embodiment depicted in this
figure, the complete periphery of the SubQ ICD may be manufactured to
have a slightly flattened perspective with rounded edges to accommodate
the placement of the electrodes 28. The electrodes 28 are welded to
housing 15 (to preserve hermaticity) and are connected via wires (not
shown) to electronic circuitry (described herein below) inside housing
15. Electrodes 28 may be constructed of flat plates, or alternatively,
may be spiral electrodes as described in U.S. Pat. No. 6,512,940
"Subcutaneous Spiral Electrode for Sensing Electrical Signals of the
Heart" to Brabec, et al and mounted in a non-conductive surround shroud
as described in U.S. Pat. No. 6,522,915 "Surround Shroud Connector and
Electrode Housings for a Subcutaneous Electrode Array and Leadless ECGs"
to Ceballos, et al and U.S. Pat. No. 6,622,046 "Subcutaneous Sensing
Feedthrough/Electrode Assembly" to Fraley, et al, all incorporated herein
by reference in their entireties. The electrodes 28 of FIG.2 can be
positioned to form orthogonal or equilateral signal vectors, for example.

[0047]The electronic circuitry employed in subcutaneous device 14 can take
any of the known forms that detect a tachyarrhythmia from the sensed ECG
and provide cardioversion/defibrillation shocks as well as post-shock
pacing as needed while the heart recovers. A simplified block diagram of
such circuitry adapted to function employing the first and second
cardioversion-defibrillation electrodes as well as the ECG sensing and
pacing electrodes described herein below is set forth in FIG. 3. It will
be understood that the simplified block diagram does not show all of the
conventional components and circuitry of such devices including digital
clocks and clock lines, low voltage power supply and supply lines for
powering the circuits and providing pacing pulses or telemetry circuits
for telemetry transmissions between the device 14 and external programmer
20.

[0048]FIG. 3 is an exemplary schematic diagram of electronic circuitry
within a hermetically sealed housing of a subcutaneous device according
to an embodiment of the present invention. As illustrated in FIG. 3,
subcutaneous device 14 includes a low voltage battery 153 coupled to a
power supply (not shown) that supplies power to the circuitry of the
subcutaneous device 14 and the pacing output capacitors to supply pacing
energy in a manner well known in the art. The low voltage battery 153 may
be formed of one or two conventional LiCFx, LiMnO2 or Lil2
cells, for example. The subcutaneous device 14 also includes a high
voltage battery 112 that may be formed of one or two conventional LiSVO
or LiMnO2 cells. Although two both low voltage battery and a high
voltage battery are shown in FIG. 3, according to an embodiment of the
present invention, the device 14 could utilize a single battery for both
high and low voltage uses.

[0049]Further referring to FIG. 3, subcutaneous device 14 functions are
controlled by means of software, firmware and hardware that cooperatively
monitor the ECG, determine when a cardioversion-defibrillation shock or
pacing is necessary, and deliver prescribed cardioversion-defibrillation
and pacing therapies. The subcutaneous device 14 may incorporate
circuitry set forth in commonly assigned U.S. Pat. No. 5,163,427
"Apparatus for Delivering Single and Multiple Cardioversion and
Defibrillation Pulses" to Keimel and U.S. Pat. No. 5,188,105 "Apparatus
and Method for Treating a Tachyarrhythmia" to Keimel for selectively
delivering single phase, simultaneous biphasic and sequential biphasic
cardioversion-defibrillation shocks typically employing ICD IPG housing
electrodes 28 coupled to the COMMON output 123 of high voltage output
circuit 140 and cardioversion-defibrillation electrode 24 disposed
posterially and subcutaneously and coupled to the HVI output 113 of the
high voltage output circuit 140. Outputs 132 of FIG. 3 is coupled to
sense electrode 26.

[0050]The cardioversion-defibrillation shock energy and capacitor charge
voltages can be intermediate to those supplied by ICDs having at least
one cardioversion-defibrillation electrode in contact with the heart and
most AEDs having cardioversion-defibrillation electrodes in contact with
the skin. The typical maximum voltage necessary for ICDs using most
biphasic waveforms is approximately 750 Volts with an associated maximum
energy of approximately 40 Joules. The typical maximum voltage necessary
for AEDs is approximately 2000-5000 Volts with an associated maximum
energy of approximately 200-360 Joules depending upon the model and
waveform used. The subcutaneous device 14 of the present invention uses
maximum voltages in the range of about 300 to approximately 1000 Volts
and is associated with energies of approximately 25 to 150 joules or
more. The total high voltage capacitance could range from about 50 to
about 300 microfarads. Such cardioversion-defibrillation shocks are only
delivered when a malignant tachyarrhythmia, e.g., ventricular
fibrillation is detected through processing of the far field cardiac ECG
employing the detection algorithms as described herein below.

[0051]In FIG. 3, sense amp 190 in conjunction with pacer/device timing
circuit 178 processes the far field ECG sense signal that is developed
across a particular ECG sense vector defined by a selected pair of the
subcutaneous electrodes 24, 26 and 28, or, optionally, a virtual signal
(i.e., a mathematical combination of two vectors) if selected. The
selection of the sensing electrode pair is made through the switch
matrix/MUX 191 in a manner to provide the most reliable sensing of the
ECG signal of interest, which would be the R wave for patients who are
believed to be at risk of ventricular fibrillation leading to sudden
death. The far field ECG signals are passed through the switch matrix/MUX
191 to the input of the sense amplifier 190 that, in conjunction with
pacer/device timing circuit 178, evaluates the sensed EGM. Bradycardia,
or asystole, is typically determined by an escape interval timer within
the pacer timing circuit 178 and/or the control circuit 144. Pace Trigger
signals are applied to the pacing pulse generator 192 generating pacing
stimulation when the interval between successive R-waves exceeds the
escape interval. Bradycardia pacing is often temporarily provided to
maintain cardiac output after delivery of a cardioversion-defibrillation
shock that may cause the heart to slowly beat as it recovers back to
normal function. Sensing subcutaneous far field signals in the presence
of noise may be aided by the use of appropriate denial and extensible
accommodation periods as described in U.S. Pat. No. 6,236,882 "Noise
Rejection for Monitoring ECGs" to Lee, et al and incorporated herein by
reference in its' entirety.

[0052]Detection of a malignant tachyarrhythmia is determined in the
control circuit 144 as a function of the intervals between R-wave sense
event signals that are output from the pacer/device timing 178 and sense
amplifier circuit 190 to the timing and control circuit 144. It should be
noted that the present invention utilizes not only interval based signal
analysis method but also supplemental sensors and morphology processing
method and apparatus as described herein below.

[0053]Supplemental sensors such as tissue color, tissue oxygenation,
respiration, patient activity and the like may be used to contribute to
the decision to apply or withhold a defibrillation therapy as described
generally in U.S. Pat. No. 5,464,434 "Medical Interventional Device
Responsive to Sudden Hemodynamic Change" to Alt and incorporated herein
by reference in its entirety. Sensor processing block 194 provides sensor
data to microprocessor 142 via data bus 146. Specifically, patient
activity and/or posture may be determined by the apparatus and method as
described in U.S. Pat. No. 5,593,431 "Medical Service Employing Multiple
DC Accelerometers for Patient Activity and Posture Sensing and Method" to
Sheldon and incorporated herein by reference in its entirety. Patient
respiration may be determined by the apparatus and method as described in
U.S. Pat. No. 4,567,892 "Implantable Cardiac Pacemaker" to Plicchi, et al
and incorporated herein by reference in its entirety. Patient tissue
oxygenation or tissue color may be determined by the sensor apparatus and
method as described in U.S. Pat. No. 5,176,137 to Erickson, et al and
incorporated herein by reference in its entirety. The oxygen sensor of
the '137 patent may be located in the subcutaneous device pocket or,
alternatively, located on the lead 18 to enable the sensing of contacting
or near-contacting tissue oxygenation or color.

[0054]Certain steps in the performance of the detection algorithm criteria
are cooperatively performed in microcomputer 142, including
microprocessor, RAM and ROM, associated circuitry, and stored detection
criteria that may be programmed into RAM via a telemetry interface (not
shown) conventional in the art. Data and commands are exchanged between
microcomputer 142 and timing and control circuit 144, pacer
timing/amplifier circuit 178, and high voltage output circuit 140 via a
bidirectional data/control bus 146. The pacer timing/amplifier circuit
178 and the control circuit 144 are clocked at a slow clock rate. The
microcomputer 142 is normally asleep, but is awakened and operated by a
fast clock by interrupts developed by each R-wave sense event, on receipt
of a downlink telemetry programming instruction or upon delivery of
cardiac pacing pulses to perform any necessary mathematical calculations,
to perform tachycardia and fibrillation detection procedures, and to
update the time intervals monitored and controlled by the timers in
pacer/device timing circuitry 178.

[0055]When a malignant tachycardia is detected, high voltage capacitors
156, 158, 160, and 162 are charged to a pre-programmed voltage level by a
high-voltage charging circuit 164. It is generally considered inefficient
to maintain a constant charge on the high voltage output capacitors 156,
158, 160, 162. Instead, charging is initiated when control circuit 144
issues a high voltage charge command HVCHG delivered on line 145 to high
voltage charge circuit 164 and charging is controlled by means of
bidirectional control/data bus 166 and a feedback signal VCAP from the HV
output circuit 140. High voltage output capacitors 156, 158, 160 and 162
may be of film, aluminum electrolytic or wet tantalum construction.

[0056]The negative terminal of high voltage battery 112 is directly
coupled to system ground. Switch circuit 114 is normally open so that the
positive terminal of high voltage battery 112 is disconnected from the
positive power input of the high voltage charge circuit 164. The high
voltage charge command HVCHG is also conducted via conductor 149 to the
control input of switch circuit 114, and switch circuit 114 closes in
response to connect positive high voltage battery voltage EXT B+ to the
positive power input of high voltage charge circuit 164. Switch circuit
114 may be, for example, a field effect transistor (FET) with its
source-to-drain path interrupting the EXT B+ conductor 118 and its gate
receiving the HVCHG signal on conductor 145. High voltage charge circuit
164 is thereby rendered ready to begin charging the high voltage output
capacitors 156, 158, 160, and 162 with charging current from high voltage
battery 112.

[0057]High voltage output capacitors 156, 158, 160, and 162 may be charged
to very high voltages, e.g., 300-1000V, to be discharged through the body
and heart between the electrode pair of subcutaneous
cardioversion-defibrillation electrodes 113 and 123. The details of the
voltage charging circuitry are also not deemed to be critical with regard
to practicing the present invention; one high voltage charging circuit
believed to be suitable for the purposes of the present invention is
disclosed. High voltage capacitors 156, 158, 160 and 162 may be charged,
for example, by high voltage charge circuit 164 and a high frequency,
high-voltage transformer 168 as described in detail in commonly assigned
U.S. Pat. No. 4,548,209 "Energy Converter for Implantable Cardioverter"
to Wielders, et al. Proper charging polarities are maintained by diodes
170, 172, 174 and 176 interconnecting the output windings of high-voltage
transformer 168 and the capacitors 156, 158, 160, and 162. As noted
above, the state of capacitor charge is monitored by circuitry within the
high voltage output circuit 140 that provides a VCAP, feedback signal
indicative of the voltage to the timing and control circuit 144. Timing
and control circuit 144 terminates the high voltage charge command HVCHG
when the VCAP signal matches the programmed capacitor output voltage,
i.e., the cardioversion-defibrillation peak shock voltage.

[0058]Control circuit 144 then develops first and second control signals
NPULSE 1 and NPULSE 2, respectively, that are applied to the high voltage
output circuit 140 for triggering the delivery of cardioverting or
defibrillating shocks. In particular, the NPULSE 1 signal triggers
discharge of the first capacitor bank, comprising capacitors 156 and 158.
The NPULSE 2 signal triggers discharge of the first capacitor bank and a
second capacitor bank, comprising capacitors 160 and 162. It is possible
to select between a plurality of output pulse regimes simply by modifying
the number and time order of assertion of the NPULSE 1 and NPULSE 2
signals. The NPULSE 1 signals and NPULSE 2 signals may be provided
sequentially, simultaneously or individually. In this way, control
circuitry 144 serves to control operation of the high voltage output
stage 140, which delivers high energy cardioversion-defibrillation shocks
between the pair of the cardioversion-defibrillation electrodes 113 and
123 coupled to the HV-1 and COMMON output as shown in FIG. 3.

[0059]Thus, subcutaneous device 14 monitors the patient's cardiac status
and initiates the delivery of a cardioversion-defibrillation shock
through the cardioversion-defibrillation electrodes 24 and 28 in response
to detection of a tachyarrhythmia requiring cardioversion-defibrillation.
The high HVCHG signal causes the high voltage battery 112 to be connected
through the switch circuit 114 with the high voltage charge circuit 164
and the charging of output capacitors 156, 158, 160, and 162 to commence.
Charging continues until the programmed charge voltage is reflected by
the VCAP signal, at which point control and timing circuit 144 sets the
HVCHG signal low terminating charging and opening switch circuit 114.
Typically, the charging cycle takes only fifteen to twenty seconds, and
occurs very infrequently. The subcutaneous device 14 can be programmed to
attempt to deliver cardioversion shocks to the heart in the manners
described above in timed synchrony with a detected R-wave or can be
programmed or fabricated to deliver defibrillation shocks to the heart in
the manners described above without attempting to synchronize the
delivery to a detected R-wave. Episode data related to the detection of
the tachyarrhythmia and delivery of the cardioversion-defibrillation
shock can be stored in RAM for uplink telemetry transmission to an
external programmer as is well known in the art to facilitate in
diagnosis of the patient's cardiac state. A patient receiving the device
14 on a prophylactic basis would be instructed to report each such
episode to the attending physician for further evaluation of the
patient's condition and assessment for the need for implantation of a
more sophisticated ICD.

[0060]Subcutaneous device 14 desirably includes telemetry circuit (not
shown in FIG. 3), so that it is capable of being programmed by means of
external programmer 20 via a 2-way telemetry link 22 (shown in FIG. 1).
Uplink telemetry allows device status and diagnostic/event data to be
sent to external programmer 20 for review by the patient's physician.
Downlink telemetry allows the external programmer via physician control
to allow the programming of device function and the optimization of the
detection and therapy for a specific patient. Programmers and telemetry
systems suitable for use in the practice of the present invention have
been well known for many years. Known programmers typically communicate
with an implanted device via a bidirectional radio-frequency telemetry
link, so that the programmer can transmit control commands and
operational parameter values to be received by the implanted device, so
that the implanted device can communicate diagnostic and operational data
to the programmer. Programmers believed to be suitable for the purposes
of practicing the present invention include the Models 9790 and
CareLink® programmers, commercially available from Medtronic, Inc.,
Minneapolis, Minn.

[0061]Various telemetry systems for providing the necessary communications
channels between an external programming unit and an implanted device
have been developed and are well known in the art. Telemetry systems
believed to be suitable for the purposes of practicing the present
invention are disclosed, for example, in the following U.S. patents: U.S.
Pat. No. 5,127,404 to Wyborny et al. entitled "Telemetry Format for
Implanted Medical Device"; U.S. Pat. No. 4,374,382 to Markowitz entitled
"Marker Channel Telemetry System for a Medical Device"; and U.S. Pat. No.
4,556,063 to Thompson et al. entitled "Telemetry System for a Medical
Device". The Wyborny et al. '404, Markowitz '382, and Thompson et al.
'063 patents are commonly assigned to the assignee of the present
invention, and are each hereby incorporated by reference herein in their
respective entireties.

[0062]FIG. 4 is a schematic diagram of signal processing aspects of a
subcutaneous device according to an exemplary embodiment of the present
invention. The transthoracic ECG signal (ECG1) detected between the
distal electrode 26 of subcutaneous lead 18 and one of electrodes 28
positioned on the subcutaneous device 14 are amplified and bandpass
filtered (2.5-105 Hz) by pre-amplifiers 202 and 206 located in Sense Amp
190 of FIG. 3. The amplified EGM signals are directed to A/D converters
210 and 212, which operate to sample the time varying analog EGM signal
and digitize the sampled points. The digital output of A/D converters 210
and 212 are applied to temporary buffers/control logic, which shifts the
digital data through its stages in a FIFO manner under the control of
Pacer/Device Timing block 178 of FIG. 3. Virtual Vector block 226 selects
one housing-based ECG signal (ECG2) from any pair of electrodes 28 as
described, for example, in U.S. Pat. No. 5,331,966 "Subcutaneous
Multi-Electrode Sensing System, Method and Pacer" to Bennett, et al or,
alternatively, generates a virtual vector signal under control of
Microprocessor 142 and Control block 144 as described in U.S. Pat. No.
6,505,067 "System and Method for Deriving Virtual ECG or EGM Signal" to
Lee, et al; both patents incorporated herein by reference in their
entireties. ECG1 and ECG2 vector selection may be selected by the
patient's physician and programmed via telemetry link 22 from programmer
20.

[0063]According to an embodiment of the present invention, in order to
automatically select the preferred ECG vector set, it is necessary to
have an index of merit upon which to rate the quality of the signal.
"Quality" is defined as the signal's ability to provide accurate heart
rate estimation and accurate morphological waveform separation between
the patient's usual sinus rhythm and the patient's ventricular
tachyarrhythmia.

[0065]Automatic vector selection might be done at implantation or
periodically (daily, weekly, monthly) or both. At implant, automatic
vector selection may be initiated as part of an automatic device turn-on
procedure that performs such activities as measure lead impedances and
battery voltages. The device turn-on procedure may be initiated by the
implanting physician (e.g., by pressing a programmer button) or,
alternatively, may be initiated automatically upon automatic detection of
device/lead implantation. The turn-on procedure may also use the
automatic vector selection criteria to determine if ECG vector quality is
adequate for the current patient and for the device and lead position,
prior to suturing the subcutaneous device 14 device in place and closing
the incision. Such an ECG quality indicator would allow the implanting
physician to maneuver the device to a new location or orientation to
improve the quality of the ECG signals as required. The preferred ECG
vector or vectors may also be selected at implant as part of the device
turn-on procedure. The preferred vectors might be those vectors with the
indices that maximize rate estimation and detection accuracy. There may
also be an a priori set of vectors that are preferred by the physician,
and as long as those vectors exceed some minimum threshold, or are only
slightly worse than some other more desirable vectors, the a priori
preferred vectors are chosen. Certain vectors may be considered nearly
identical such that they are not tested unless the a priori selected
vector index falls below some predetermined threshold.

[0066]Depending upon metric power consumption and power requirements of
the device, the ECG signal quality metric may be measured on the range of
vectors (or alternatively, a subset) as often as desired. Data may be
gathered, for example, on a minute, hourly, daily, weekly or monthly
basis. More frequent measurements (e.g., every minute) may be averaged
over time and used to select vectors based upon susceptibility of vectors
to occasional noise, motion noise, or EMI, for example.

[0067]Alternatively, the subcutaneous device 14 may have an
indicator/sensor of patient activity (piezo-resistive, accelerometer,
impedance, or the like) and delay automatic vector measurement during
periods of moderate or high patient activity to periods of minimal to no
activity. One representative scenario may include testing/evaluating ECG
vectors once daily or weekly while the patient has been determined to be
asleep (using an internal clock (e.g., 2:00 am) or, alternatively, infer
sleep by determining the patient's position (via a 2- or 3-axis
accelerometer) and a lack of activity).

[0068]If infrequent automatic, periodic measurements are made, it may also
be desirable to measure noise (e.g., muscle, motion, EMI, etc.) in the
signal and postpone the vector selection measurement when the noise has
subsided.

[0069]Subcutaneous device 14 may optionally have an indicator of the
patient's posture (via a 2- or 3-axis accelerometer). This sensor may be
used to ensure that the differences in ECG quality are not simply a
result of changing posture/position. The sensor may be used to gather
data in a number of postures so that ECG quality may be averaged over
these postures or, alternatively, selected for a preferred posture.

[0070]In the preferred embodiment, vector quality metric calculations
would occur a number of times over approximately 1 minute, once per day,
for each vector. These values would be averaged for each vector over the
course of one week. Averaging may consist of a moving average or
recursive average depending on time weighting and memory considerations.
In this example, the preferred vector(s) would be selected once per week.

[0071]Continuing with FIG. 4, a diagnostic channel 228 receives a
programmable selected ECG signal from the housing based subcutaneous
electrodes and the transthoracic ECG from the distal electrode 26 on lead
18. Block 238 compresses the digital data, the data is applied to
temporary buffers/control logic 218 which shifts the digital data through
its stages in a FIFO manner under the control of Pacer/Device Timing
block 178 of FIG. 3, and the data is then stored in SRAM block 244 via
direct memory access block 242.

[0072]The two selected ECG signals (ECG1 and ECG2) are additionally used
to provide R-wave interval sensing via ECG sensing block 230. IIR notch
filter block 246 provides 50/60 Hz notch filtering. A rectifier and
auto-threshold block 248 provides R-wave event detection as described in
U.S. Pat. No. 5,117,824 "Apparatus for Monitoring Electrical Physiologic
Signals" to Keimel, et al; publication WO2004023995 "Method and Apparatus
for Cardiac R-wave Sensing in a Subcutaneous ECG Waveform" to Cao, et al
and U.S. Publication No. 2004/0260350 "Automatic EGM Amplitude
Measurements During Tachyarrhythmia Episodes" to Brandstetter, et al, all
incorporated herein by reference in their entireties. The rectifier of
block 248 performs full wave rectification on the amplified, narrowband
signal from bandpass filter 246. A programmable fixed threshold
(percentage of peak value), a moving average or, more preferably, an
auto-adjusting threshold is generated as described in the '824 patent or
'350 publication. In these references, following a detected
depolarization, the amplifier is automatically adjusted so that the
effective sensing threshold is set to be equal to a predetermined portion
of the amplitude of the sensed depolarization, and the effective sensing
threshold decays thereafter to a lower or base-sensing threshold. A
comparator in block 248 determines signal crossings from the rectified
waveform and auto-adjusting threshold signal. A timer block 250 provides
R-wave to R-wave interval timing for subsequent arrhythmia detection (to
be described herein below). The heart rate estimation is derived from the
last 12 R-R intervals (e.g., by a mean, trimmed mean, or median, for
example), with the oldest data value being removed as a new data value is
added.

[0073]FIG. 5 is a schematic diagram of a rectifier and auto-threshold unit
in a subcutaneous device according to an embodiment of the present
invention. Waveform 402 depicts a typical subcutaneous ECG waveform and
waveform 404 depicts the same waveform after filtering and rectification.
A time dependant threshold 406 allows a more sensitive sensing threshold
temporally with respect to the previous sensed R-wave. Sensed events 408
indicate when the rectified ECG signal 404 exceeds the auto-adjusting
threshold and a sensed event has occurred.

[0074]Returning to FIG. 4, the subcutaneous ECG signal (ECG1) is applied
to ECG morphology block 232, filtered by a 2-pole 23 Hz low pass filter
252 and evaluated by DSP microcontroller 254 under control of program
instructions stored in System Instruction RAM 258. ECG morphology is used
for subsequent rhythm detection/determination (to be described herein
below).

[0075]FIG. 6 is a state diagram of detection of arrhythmias in a medical
device according to an embodiment of the present invention. As
illustrated in FIG. 6, during normal operation, the device 14 is in a not
concerned state 302, described in more detail herein below, during which
R-wave intervals are being evaluated to identify periods of rapid rates
and/or the presence of asystole. Upon detection of short R-wave intervals
simultaneously in both ECG leads, indicative of an event that, if
confirmed, may require the delivery of therapy, the device 14 transitions
from the not concerned state 302 to a concerned state 304, described in
more detail herein below. In the concerned state 304 the device 14
evaluates a predetermined window of ECG signals to determine the
likelihood that the signal is corrupted with noise and to discriminate
rhythms requiring shock therapy from those that do not require shock
therapy, using a combination of R-wave intervals and ECG signal
morphology information.

[0076]If a rhythm requiring shock therapy continues to be detected while
in the concerned state 304, the device 14 transitions from the concerned
state 304 to an armed state 306, described in more detail herein below.
If a rhythm requiring shock therapy is no longer detected while the
device is in the concerned state 304 and the R-wave intervals are
determined to no longer be short, the device 14 returns to the not
concerned state 302. However, if a rhythm requiring shock therapy is no
longer detected while the device is in the concerned state 304, but the
R-wave intervals continue to be detected as being short, processing
continues in the concerned state 304.

[0077]In the armed state 306, the device 14 charges the high voltage
shocking capacitors and continues to monitor R-wave intervals and ECG
signal morphology for spontaneous termination. If spontaneous termination
of the rhythm requiring shock therapy occurs, the device 14 returns to
the not concerned state 302. If the rhythm requiring shock therapy is
still determined to be occurring once the charging of the capacitors is
completed, the device 14 transitions from the armed state 306 to a shock
state 308, described in more detail herein below.

[0078]In the shock state 308, the device 14 delivers a shock and returns
to the armed state 306 to evaluate the success of the therapy delivered.

[0079]FIGS. 7A-7I are flow charts of a method for detecting arrhythmias in
a subcutaneous device according to an embodiment of the present
invention. As illustrated in FIG. 7A, device 14 continuously evaluates
the two channels ECG1 and ECG2 associated with two predetermined
electrode vectors to when sensed events occur. For example, the electrode
vectors for the two channels ECG1 and ECG2 may include a horizontal
vector selected between two of the electrodes 28 (ECG2) located along the
housing of the device 14 as one electrode vector, while the other
electrode vector is a front to back vector selected between the distal
electrode 26 (ECG1) and one of the subcutaneous electrodes 28, for
example. The input signal from each channel ECG1 and ECG2 is
pre-processed and rectified, and an adaptive auto-adjusting threshold is
then applied. According to an embodiment of the present invention, a
sensed event is determined to have occurred, for example, whenever the
rising edge of the filtered ECG crosses the threshold.

[0080]FIG. 8 is a graphical representation of sensing of cardiac activity
according to an embodiment of the present invention. In particular, the
present invention utilizes an adaptive auto-adjusting threshold 401
during the R-wave sensing of Block 322 that includes a first threshold
level 403, a second threshold level 405, a third threshold level 407 and
a fourth threshold level 409. An example of an auto-adjusting threshold
is described, for example, in commonly assigned U.S. Patent Application
Publication No. 2004/0049120, to Cao et al., filed Sep. 11, 2002,
incorporated herein by reference in its entirety. Once there is a sensed
event, which occurs whenever the rising edge of the rectified filtered
ECG 411 crosses the threshold level, in this case threshold 403,
indicated by marker 410, the threshold 401 is adjusted to the second
threshold level 405, which is a first predetermined percentage of a peak
amplitude 412 of the rectified filtered ECG 411, such as 65 percent, for
example.

[0081]A blanking period 414 (nominally 150 ms, for example) prevents
double counting of R-waves. During blanking period 414, the threshold 401
continues to track the predetermined percentage of rectified filtered ECG
411 until peak 412 is detected. Threshold 401 is held at the second
threshold level 405 during a threshold hold time period 416 (nominally
100 ms, for example) starting from the peak 412 location to prevent
T-wave oversensing by delaying the linear decay. Threshold 401 then
decays at a first predetermined rate, such as 35% of peak 412 per second,
for example, until threshold 401 reaches the third threshold level 407,
which is a second predetermined percentage of peak amplitude 412
(nominally 30%, for example). Threshold 401 is held at the third
threshold level 407 until a step drop time 418 from the sensed event 410
(1.5 sec, for example) has expired. Once the step drop time 418 has
expired, the threshold 401 is instantaneously set at the fourth threshold
level 409 and begins to decay at a second predetermined rate, such as 20%
of peak 412 per second, for example. The threshold 401 continues to decay
linearly at the second predetermined rate until the threshold 401 reaches
the first threshold level 403. At no time can the threshold 401 become
less than the first threshold level 403.

[0082]The step drop time 418 allows abrupt adjustment of the threshold 401
in order to accommodate sensing of sudden reductions in R-wave
amplitudes. The second predetermined rate associated with the linear
decay is set at a rate that prevents oversensing of P-waves while
maintaining adequate decay for sensing sudden drops in R-waves. If, at
any time throughout this threshold adjustment process, a sensed event
re-occurs outside blanking period 414, then the threshold 401 is adjusted
to the second threshold level 405, and the threshold adjustment process
is repeated.

[0083]According to an embodiment of the present invention, the nominal
settings for the R-wave detector parameters may be set, for example, with
the first threshold level being 25 microvolts, the second threshold
level, third threshold level and fourth threshold level being set as 65,
30 and 20 percent of the peak amplitude 412, respectively, blanking
period 414 being set as 150 milliseconds, threshold hold time 416 being
set as 100 milliseconds, and a maximum threshold level being 650
microvolts. These nominal settings may differ between the anterior
housing-based bipolar ECG and the front to back ECG in order to account
for the expected difference in amplitude and noise characteristics for
those vectors.

[0084]The R-wave sensing described above is applied to each ECG channel
ECG1 and ECG2 independently. According to the present invention, sensing
of ventricular events on either channel will trigger execution of state
machine in states 1 and 4. During states 2 and 3, R-wave sensing
continues but state machine is triggered every predetermined number of
seconds, as described below.

[0085]Returning to FIG. 7A, a buffer of the most recent 12 R-R intervals
obtained during R-wave processing using the sensing scheme of FIG. 8,
described above, for example, is independently maintained for each of the
two sensing channels ECG1 and ECG2. When the next sensed R-wave is
obtained, Block 322, which initially would be the 12th R-wave
interval, a heart rate estimate is determined, Block 323, using a metric
of heart rate, such as the mean, trimmed mean, or median of the RR
intervals, for example. According to an embodiment of the present
invention, the 9th fastest beat of the 12 beats on a beat by beat
basis is utilized as the heart rate metric. Using the 9th fastest
beat provides an estimate of heart rate that is less susceptible to
oversensing while maintaining reasonable sensitivity to short R-R
intervals as in the case of VT/VF. If the buffer of 12 R-R intervals
contains any unknown R-R intervals (i.e., because the buffer is not yet
filled) the initial estimate of heart rate is unknown.

[0086]Once the heart rate estimate is obtained using the heart rate
metric, a determination is made as to whether asystole is detected for
either channel, ECG1 or ECG 2, Block 324. According to an embodiment of
the present invention, asystole is detected for the channel, for example,
either by determining whether one of the 12 R-R intervals is greater than
a predetermined time period, such as three seconds, for example, or if
the time since the most recently sensed R wave exceeds a predetermined
time period, such as three seconds, for example. The latter can occur if
an R-wave is sensed, for example, in one channel ECG1, but the other
channel ECG2 has not had an R-wave sense in three or more seconds. If
asystole is detected for either of the two channels ECG1 or ECG2, the
current 12 R-R intervals for channels that are determined to be in
asystole are cleared from the buffers, Block 325, and the process
continues by determining whether the current heart rate estimate is
reliable for both channels ECG1 and ECG2, Block 328, described below.

[0087]If asystole is not detected for either channel ECG1 and ECG2, NO in
Block 324, a determination is made independently for both channels ECG1
and ECG2 as to whether the current heart rate estimate for both channels
ECG1 and ECG2 is reliable, Block 328. According to an embodiment of the
present invention, the current heart rate estimate for each of the two
channels ECG1 and ECG2 is determined not to be reliable, No in Block 328,
if either there are unknown or cleared entries in the buffer for that
channel, or if a predetermined number of the sensed R-waves associated
with the current 12 R-R intervals for that channel was sensed at the
minimum sensing threshold level, i.e., the first threshold level 403 of
FIG. 8, for example, and if the current heart rate estimate for the
channel is less than the programmed heart rate threshold. According to
one embodiment, the predetermined number of sensed R-waves that must be
sensed at the minimum threshold is set at two, for example. In addition,
the programmed heart rate threshold may be within a range of 150 to 240
beats per minute, and is nominally set at 180 beats per minute, for
example. It is understood that while the processing is described using a
buffer of 12 R-R intervals, any number of intervals and predetermined
number of sensed R-waves that must be sensed at the minimum threshold may
be utilized.

[0088]If the above analysis does not determine that both of the channels
are reliable, No in Block 328, a determination is made as to whether just
one of the channels was unreliable or if both channels were unreliable,
Block 330. If both channels are determined to be unreliable, the current
12 R-R intervals for both channels ECG1 and ECG2 are cleared from the
buffers, Block 326, and the next R-sense is obtained for each channel,
Block 322 using the sensing scheme of FIG. 8, described above, so that a
new heart rate estimate is determined, Block 323, based on the new R-R
intervals.

[0089]If only one channel is determined to unreliable, the value for the
heart rate estimate for both channels is set to the current heart rate
estimate for the channel determined to be reliable, Block 332. Once
either both channels are determined to be reliable, YES in Block 328, or
only one of the two channels is determined to be unreliable and therefore
the heart rate estimate for both channels is set to the current heart
rate estimate for the channel determined to be reliable, Block 332, the
final heart rate estimate is determined for each channel ECG1 and ECG2
based on those results, Block 334, i.e., the heart rate estimate for each
channel is set equal to their respective heart rate estimates determined
in Block 323, or both are set equal to the heart rate estimate associated
with the channel determined to be reliable, Block 332. A determination is
then made as to whether the final heart rate estimates for both channels
is greater than a predetermined VT/VF threshold, Block 336. According to
an embodiment of the present invention, the predetermined VT/VF threshold
of Block 336 is set at 180 bpm, for example, although any desired
threshold could be utilized.

[0090]If the final heart rate estimates for one or both channels is not
greater than the predetermined VT/VF threshold, the buffer containing the
12 R-R intervals for the channel not greater than the predetermined VT/VF
threshold is updated by removing the first R-sense, shifting the
remaining eleven R-sense samples back so that the second R-sense becomes
the first R-sense, and so forth, and inserting the next detected R-sense,
Block 322, as the twelfth R-sense for each corresponding channel ECG1 and
ECG2. A new current heart rate estimate is then determined, Block 323.
Once the final heart rate estimates for both channels is greater than the
predetermined VT/VF threshold, Yes in Block 336, the process transitions
from the not concerned state 302 to the concerned state 304.

[0091]According to the present invention, upon transition from the not
concerned state 302 to the concerned state 304, Block 305, a most recent
window of ECG data from both channels ECG1 and ECG2 are utilized, such as
three seconds, for example, so that processing is triggered in the
concerned state 304 by a three-second timeout, rather than by the sensing
of an R-wave, which is utilized when in the not concerned state 302,
described above. It is understood that while the processing is described
as being triggered over a three second period, other times periods for
the processing time utilized when in the concerned state 304 may be
chosen, but should preferably be within a range of 0.5 to 10 seconds. As
a result, although sensing of individual R-waves continues to occur in
both channels ECG1 and ECG2 when in the concerned state 304, and the
buffer of 12 R-R intervals continues to be updated, the opportunities for
changing from the concerned state 304 to another state and the estimates
of heart rate only occur once the three-second timer expires. Upon
initial entry to the concerned state 304, it is advantageous to process
the most recent three-seconds of ECG data, i.e., ECG data for the three
seconds leading up to the transition to the concerned state 304. This
requires a continuous circular buffering of the most recent three seconds
of ECG data even while in the not concerned state 302.

[0092]As described in detail below, while in the concerned state 304, the
present invention determines how sinusoidal and how noisy the signals are
in order to determine the likelihood that a ventricular fibrillation (VF)
or fast ventricular tachycardia (VT) event is taking place, since the
more sinusoidal and low noise the signal is, the more likely a VT/VF
event is taking place. As illustrated in FIG. 7B, once the device
transitions from the not concerned state 302 to the concerned state 304,
Block 305, a buffer for each of the two channels ECG1 and ECG2 for
storing classifications of 3-second segments of data as "shockable" or
"non-shockable" is cleared. Processing of signals of the two channels
ECG1 and ECG2 while in the concerned state 304 is then triggered by the
three second time period, rather than by the sensing of an R-wave
utilized during the not concerned state 302, described above.

[0093]Once the three second time interval has expired, YES in Block 341,
morphology characteristics of the signal during the three second time
interval for each channel are utilized to determine whether the signals
are likely corrupted by noise artifacts and to characterize the
morphology of the signal as "shockable" or "not shockable". For example,
using the signals associated with the three second time interval, a
determination is made for each channel ECG1 and ECG 2 as to whether the
channel is likely corrupted by noise, Block 342, and a determination is
then made as to whether both channels ECG1 and ECG2 are corrupted by
noise, Block 344.

[0094]As illustrated in FIG. 7C, the determination as to whether the
signal associated with each of the channels ECG1 and ECG2 is likely
corrupted by noise, Block 342 of FIG. 7B, includes multiple sequential
noise tests that are performed on each channel ECG and ECG2. During a
first noise test, for example, a determination is made as to whether a
metric of signal energy content of the signal for the channel is within
predetermined limits, Block 380. For example, the amplitude of each
sample associated with the three second window is determined, resulting
in N sample amplitudes, from which a mean rectified amplitude is
calculated as the ratio of the sum of the rectified sample amplitudes to
the total number of sample amplitudes N for the segment. If the sampling
rate is 256 samples per second, for example, the total number of sample
amplitudes N for the three-second segment would be N=768 samples.

[0095]Once the mean rectified amplitude is calculated, a determination is
made as to whether the mean rectified amplitude is between an upper
average amplitude limit and a lower average amplitude limit, the lower
average amplitude limit being associated with asystole episodes without
artifact and the upper average amplitude limit being associated with a
value greater than what would be associated with ventricular tachycardia
and ventricular fibrillation events. According to an embodiment of the
present invention, the upper average amplitude limit is set as 1.5 mV,
and the lower average amplitude limit is set as 0.013 mV. While the
metric of signal energy content is described above as the mean rectified
amplitude, it is understood that other signal of energy contents could be
utilized.

[0096]If the determined mean rectified amplitude is not between the upper
average amplitude limit and the lower average amplitude limit, the three
second segment for that channel is identified as being likely corrupted
with noise, Block 386, and no further noise tests are initiated for that
channel's segment.

[0097]If the determined mean rectified amplitude is located between the
upper average amplitude limit and the lower average amplitude limit, a
noise to signal ratio is calculated and a determination is made as to
whether the noise to signal ratio is less than a predetermined noise to
signal threshold, Block 382. For example, the amplitude of each sample
associated with the three second window is determined, resulting in N raw
sample amplitudes. The raw signal is lowpass filtered, resulting in L
lowpass sample amplitudes. The raw mean rectified amplitude is determined
as the average of the absolute values of the raw sample amplitudes. The
lowpass mean rectified amplitude is determined as the average of the
absolute values of the lowpass sample amplitudes. Next, a highpass mean
rectified amplitude is then calculated as the difference between the raw
mean rectified amplitude and the lowpass mean rectified amplitude. The
noise to signal ratio is then determined as the ratio of the highpass
mean rectified amplitude to the lowpass mean rectified amplitude. If the
noise to signal ratio is greater than a predetermined threshold, such as
0.0703, for example, the three second segment for that channel is
identified as being likely corrupted with noise, Block 386, and no
further noise tests are initiated for the segment.

[0098]If the noise to signal ratio is less than or equal to the
predetermined threshold, a determination is made as to whether the signal
is corrupted by muscle noise, Block 384. According to an embodiment of
the present invention, the determination as to whether the signal is
corrupted by muscle noise is made by determining whether the signal
includes a predetermined number of signal inflections indicative of the
likelihood of the signal being corrupted by muscle noise, using a muscle
noise pulse count that is calculated to quantify the number of signal
inflections in the three second interval for each channel ECG1 and ECG2.
The presence of a significant number of inflections is likely indicative
of muscle noise.

[0099]FIG. 9A is a graphical representation of a determination of whether
a signal is corrupted by muscle noise according to an embodiment of the
present invention. FIG. 9B is a flowchart of a method of determining
whether a signal is corrupted by muscle noise according to an embodiment
of the present invention. For example, as illustrated in FIGS. 9A and 9B,
in order to determine a muscle noise count for the three second interval,
the raw signal 420 is applied to a first order derivative filter to
obtain a derivative signal 422, and all of the zero-crossings 424 in the
derivative signal 422 are located, Block 460. A data pair corresponding
to the data points immediately prior to and subsequent to the zero
crossings 424, points 426 and 428 respectively, for each crossing is
obtained. The value of the data point in each data pair with smaller
absolute value is zeroed in order to allow a clear demarcation of each
pulse when a rectified signal 430 is derived from the derivative signal
422 with zeroed zero-crossing points 432.

[0100]A pulse amplitude threshold Td, for determining whether the
identified inflection is of a significant amplitude to be identified as
being associated with muscle noise, is determined, Block 462, by dividing
the rectified signal from the three second segment into equal
sub-segments 434, estimating a local maximum amplitude 436-442 for each
of the sub-segments 434, and determining whether the local amplitudes
436-442 are less than a portion of the maximum amplitude, which is
maximum amplitude 440 in the example of FIG. 9, for the whole three
second segment. If the local maximum amplitude is less than the portion
of the maximum amplitude for the whole three second segment, the local
maximum amplitude is replaced by the maximum for the whole three second
segment for the sub-segment corresponding to that local maximum
amplitude.

[0101]It is understood that while only two or less zero-crossing points
are shown as being located within the sub-segments in the illustration of
FIG. 9 for the sake of simplicity, in fact each of the sub-segments 434,
which have a length of approximately 750 milliseconds, will contain many
inflections, such as every 25 milliseconds, for example.

[0102]According to an embodiment of the present invention, the three
second segment is divided into four sub-segments and the local maximum
amplitudes are replaced by the maximum amplitude for the whole segment if
the local maximum amplitude is less than one fifth of the maximum
amplitude for the whole segment. Once the determination of whether to
replace the local maximum amplitudes for each of the sub-segments with
the maximum amplitude for the whole segment is completed, the pulse
amplitude threshold Td for the segment is set equal to a predetermined
fraction of the mean of the local maximum amplitudes for each of the
sub-segments. According to an embodiment of the present invention, the
pulse amplitude threshold Td for the three second segment is set equal to
one sixth of the mean of the local maximum amplitudes 436-440.

[0103]Once the pulse amplitude threshold Td has been determined, the
inflections associated with the signal for the three second segment is
classified as being of significant level to be likely indicative of noise
by determining whether the pulse amplitude threshold Td is less than a
pulse threshold, Block 464. According to an embodiment of the present
invention, the pulse threshold is set as 1 microvolt. If the pulse
amplitude threshold Td is less than the pulse threshold, the signal
strength is too small for a determination of muscle noise, and therefore
the signal is determined to be not likely corrupted by noise and
therefore the channel is determined to be not noise corrupted, Block 466.

[0104]If the pulse amplitude threshold Td is greater than or equal to the
pulse threshold, the three second segment is divided into twelve
sub-segments of 250 ms window length, the number of muscle noise pulses
in each sub-segment is counted, and both the sub-segment having the
maximum number of muscle noise pulses and the number of sub-segments
having 6 or more muscle noise pulses that are greater than a
predetermined minimum threshold is determined. Muscle noise is determined
to be present in the signal if either the maximum number of muscle noise
pulses in a single sub-segment is greater than a noise pulse number
threshold or the number of sub-segments of the twelve sub-segments having
6 or more muscle noise pulses greater than the minimum threshold is
greater than or equal to a sub-segment pulse count threshold. According
to an embodiment of the present invention, the noise pulse number
threshold is set equal to eight and the sub-segment pulse count threshold
is set equal to three.

[0105]For example, if the pulse amplitude threshold Td is greater than or
equal to the pulse threshold, No in Block 464, the maximum number of
muscle noise counts in a single sub-segment is determined, Block 468. If
the maximum number of muscle noise counts is greater than the noise pulse
number threshold, Yes in Block 470, the channel is determined to be noise
corrupted, Block 472. If the maximum number of muscle noise counts for
the channel is less than or equal to the noise pulse number threshold, No
in Block 470, the number of sub-segments of the twelve sub-segments
having 6 or more muscle noise pulses greater than the minimum threshold
is determined, Block 474, and if the number is greater than or equal to a
sub-segment pulse count threshold, Yes in Block 476, the channel is
determined to be noise corrupted, Block 472. If the number is less than
the sub-segment pulse count threshold, No in Block 476, the channel is
determined not to be noise corrupted, Block 466.

[0106]FIG. 9C is a flowchart of a method of determining whether a signal
is corrupted by muscle noise according to an embodiment of the present
invention. Since muscle noise can be present during an episode of
ventricular tachycardia, the width of the overall signal pulse waveform
is determined in order to distinguish between signals that are determined
likely to be purely noise related and signals that are both shockable
events and determined to include noise. Therefore, as illustrated in FIG.
9C, according to an embodiment of the present invention, once muscle
noise is determined to be present as a result of the muscle noise pulse
count being satisfied, No in Block 470 and Yes in Block 476, a
determination is made as to whether the signal is both noise corrupted
and shockable, Block 480.

[0107]According to an embodiment of the present invention, the
determination in Block 480 as to whether the signal is both noisy and
shockable is made, for example, by dividing the rectified signal, having
768 data points, into four sub-segments and determining a maximum
amplitude for each of the four sub-segments by determining whether a
maximum amplitude for the sub-segment is less than a portion of the
maximum amplitude for the entire rectified signal in the three second
segment. For example, a determination is made for each sub-segment as to
whether the maximum amplitude for the sub-segment is less than one fourth
of the maximum amplitude for the entire rectified signal. If less than a
portion of the maximum amplitude for the entire rectified signal in the
three second segment, the maximum amplitude for the sub-segment is set
equal to the maximum amplitude for the entire rectified signal.

[0108]A mean rectified amplitude for each of the sub-segments is
determined by dividing the sum of the rectified amplitudes for the
sub-segment by the number of samples in the sub-segment, i.e., 768/4.
Then the normalized mean rectified amplitude for each sub-segment is
determined by dividing the mean rectified amplitude for each of the
sub-segments by the peak amplitude for the sub-segment. The normalized
mean rectified amplitude for the three second segment is then determined
as the sum of the normalized mean rectified amplitudes for each
sub-segment divided by the number of sub-segments, i.e., four.

[0109]Therefore, once muscle noise is suspected as a result of the
determination of the muscle noise pulse count, the determination of Block
480 based on whether the normalized mean rectified amplitude for the
three second segment is greater than a predetermined threshold for
identifying signals that, despite being indicative of a likelihood of
being associated with noise, nevertheless are associated with a shockable
event. For example, according to an embodiment of the present invention,
a determination is made as to whether the normalized mean rectified
amplitude for the three second segment is greater than 18 microvolts. If
the normalized mean rectified amplitude for the three second segment is
less than or equal to the predetermined threshold, the channel is likely
corrupted by muscle noise and not shockable, No in Block 480, and is
therefore identified as being corrupted by noise, Block 472. If the
normalized mean rectified amplitude for the three second segment is
greater than the predetermined threshold, the channel is determined to be
likely corrupted by muscle noise and shockable, Yes in Block 480, and is
therefore identified as not to be likely corrupted by muscle noise, Block
478.

[0110]Returning to FIG. 7C, when the signal is determined to be not likely
corrupted by muscle noise, a determination is made as to whether the mean
frequency of the signal associated with the channel is less than a
predetermined mean frequency threshold, Block 388, such as 11 Hz for
example. The mean frequency of the signal during the 3 second segment for
each channel ECG 1 and ECG2 is generated, for example, by calculating the
ratio of the mean absolute amplitude of the first derivative of the 3
second segment to the mean absolute amplitude of the 3 second segment,
multiplied by a constant scaling factor. If the mean frequency is
determined to be greater than or equal to the predetermined mean
frequency threshold, No in Block 388, the three second segment for that
channel is identified as being likely corrupted with noise, Block 386. If
the mean frequency is determined to be less than the predetermined mean
frequency threshold, Yes in Block 388, the three second segment for that
channel is identified as being not noise corrupted, Block 390.

[0111]According to an embodiment of the present invention, since the mean
spectral frequency tends to be low for true ventricular fibrillation
events, moderate for organized rhythms such as sinus rhythm and
supraventricular tachycardia, for example, and high during asystole and
noise, the determination in Block 388 includes determining whether the
mean frequency is less than a predetermined upper mean frequency
threshold, such as 11 Hz (i.e., mean period T of approximately 91
milliseconds) for example, and whether the mean frequency is less than a
predetermined lower mean frequency, such as 3 Hz for example. If the mean
frequency is below a second, lower threshold, such as 3 Hz, for example,
the signal is also rejected as noise and no further noise tests are
initiated. This comparison of the mean frequency to a second lower
threshold is intended to identify instances of oversensing, resulting in
appropriate transition to the concerned state. If the mean frequency of
the signal is less than 3 Hz, it is generally not possible for the heart
rate to be greater than 180 beats per minute. In practice, it may be
advantageous to set the lower frequency threshold equal to the programmed
VT/VF detection rate, which is typically approximately 3 Hz.

[0112]Therefore, in the determination of Block 388, if the mean frequency
is determined to be either greater than or equal to the predetermined
upper mean frequency threshold or less than the lower threshold, the
three second segment for that channel is identified as being likely
corrupted with noise, Block 386. If the mean frequency is determined to
be both less than the predetermined upper mean frequency threshold and
greater than the lower threshold, the three second segment for that
channel is identified as not being noise corrupted, Block 390.

[0113]Returning to FIG. 7B, once the determination as to whether the
channels ECG1 and ECG2 are corrupted by noise is made, Block 342, a
determination is made as to whether both channels are determined to be
noise corrupted, Block 344. If the signal associated with both channels
ECG1 and ECG2 is determined to likely be corrupted by noise, both
channels are classified as being not shockable, Block 347, and therefore
a buffer for each channel ECG1 and ECG 2 containing the last three
classifications of the channel is updated accordingly. If both channels
ECG1 and ECG2 are not determined to be likely corrupted by noise, No in
Block 344, the device distinguishes between either one of the channels
being not corrupted by noise or both channels being not corrupted by
noise by determining whether noise was determined to be likely in only
one of the two channels ECG1 and ECG2, Block 346.

[0114]If noise was likely in only one of the two channels, a determination
is made whether the signal for the channel not corrupted by noise, i.e.,
the clean channel, is more likely associated with a VT event or with a VF
event by determining, for example, whether the signal for that channel
includes R-R intervals that are regular and the channel can be therefore
classified as being relatively stable, Block 348. If the R-R intervals
are determined not to be relatively stable, NO in Block 348, the signal
for that channel is identified as likely being associated with VF, which
is then verified by determining whether the signal is in a VF shock zone,
Block 350, described below. If R-R intervals for that channel are
determined to be stable, YES in Block 348, the signal is identified as
likely being associated with VT, which is then verified by determining
whether the signal is in a VT shock zone, Block 352, described below.

[0115]If noise was not likely for both of the channels, No in Block 346,
i.e., both channels are determined to be clean channels, a determination
is made whether the signal for both channels is more likely associated
with a VT event or with a VF event by determining whether the signal for
both channels includes R-R intervals that are regular and can be
therefore classified as being relatively stable, Block 356. If the R-R
intervals are determined not to be relatively stable, NO in Block 356,
the signal for both channels is identified as likely being associated
with VF, which is then verified by determining whether the signal for
each channel is in a VF shock zone, Block 360, described below. If R-R
intervals for both channels are determined to be stable, YES in Block
356, the signal is identified as likely being associated with VT, which
is then verified by determining, based on both channels, whether the
signal is in a VT shock zone, Block 352.

[0116]As illustrated in FIG. 7D, according to an embodiment of the present
invention, in order to determine whether the signal for both channels
includes R-R intervals that are regular and the channels can be therefore
classified as being relatively stable, Block 356, predetermined maximum
and minimum intervals for each channel ECG1 and ECG2 are identified,
Block 500, from the updated buffer of 12 RR-intervals, Block 342.
According to one embodiment of the present invention, the largest
RR-interval and the sixth largest RR-interval of the twelve RR-intervals
are utilized as the maximum interval and the minimum interval,
respectively.

[0117]The difference between the maximum RR-interval and the minimum
RR-interval of the 12 RR-intervals is calculated for each channel ECG1
and ECG2, Block 502, to generate a first interval difference associated
with the first channel ECG1 and a second interval difference associated
with the second channel ECG2. The smallest of the first interval
difference and the second interval difference is then identified, Block
504, and a determination is made as to whether the minimum of the first
interval difference and the second interval difference is greater than a
predetermined stability threshold, Block 506, such as 110 milliseconds,
for example.

[0118]If the minimum of the first interval difference and the second
interval difference is greater than the stability threshold, the event is
classified as an unstable event, Block 508, and a determination is made
for each channel as to whether the signal associated with the channel is
within a predetermined VF shock zone, Blocks 360 and 362 of FIG. 7B,
described below. If the minimum of the first interval difference and the
second interval difference is less than or equal to the stability
threshold, No in Block 506, the device determines which one of the
minimum RR-interval associated with the first channel ECG1 and the
minimum RR-interval associated with the second channel ECG2 is shortest,
Block 510, and determines whether the shortest minimum interval is
greater than a minimum interval threshold, Block 512, such as 200
milliseconds, for example.

[0119]If the shortest of the two minimum intervals is less than or equal
to the minimum interval threshold, the event is classified as an unstable
event, Block 508, and a determination is made for each channel as to
whether the signal associated with the channel is within a predetermined
VF shock zone, Blocks 360 and 362 of FIG. 7B, described below. If the
shortest of the two minimum intervals is greater than the minimum
interval threshold, the device determines which one of the minimum
RR-interval associated with the first channel ECG1 and the minimum
RR-interval associated with the second channel ECG2 is the greatest,
Block 514, and determines whether the maximum of the two minimum
intervals is less than or equal to a maximum interval threshold, Block
516, such as 333 milliseconds for example. If the maximum of the two
minimum intervals is greater than the maximum interval threshold, the
event is classified as an unstable event, Block 508, and a determination
is made for each channel as to whether the signal associated with the
channel is within a predetermined VF shock zone, Blocks 360 and 362 of
FIG. 7B, described below. If the maximum of the two minimum intervals is
less than or equal to the maximum interval threshold, the event is
classified as a stable event, Block 518, and a determination is made,
based on both channels ECG1 and ECG2, as to whether the signal is within
a predetermined VT shock zone, Block 358, described below.

[0120]Returning to FIG. 7B, according to an embodiment of the present
invention, during the determination of whether the signal associated with
each of the channels ECG1 and ECG2 is within the VF shock zone, Blocks
360 and 362, the VF shock zone is defined based upon a low slope content
metric and a spectral width metric for each of the two channels ECG1 and
ECG2. The low slope content metric is calculated as the ratio of the
number of data points with low slope to the total number of samples in
the 3-second segment. For example, according to an embodiment of the
present invention, the difference between successive ECG samples is
determined as an approximation of the first derivative (i.e., the slope)
of the ECG signal. In particular, as illustrated in FIG. 7E, the raw
signal for each channel is applied to a first order derivative filter to
obtain a derivative signal for the three-second segment, Block 530. The
derivative signal is then rectified, divided into four equal
sub-segments, and the largest absolute slope is estimated for each of the
four sub-segments, Block 532.

[0121]A determination is made as to whether the largest absolute slopes
are less than a portion of the overall largest absolute slope for the
whole three-second segment, Block 534, such as one-fifth of the overall
absolute slope, for example. If the largest absolute slope is less than
the portion of the overall slope, then the slope value for that
sub-segment is set equal to the overall largest absolute slope, Block
536. If the largest absolute slope is not less than the portion of the
overall slope, then the slope value for that sub-segment is set equal to
the determined largest absolute slope for the sub-segment, Block 538.

[0122]Once the slope value for each of the sub-segments has been
determined and updated by being set equal to the largest slope for the
three second segment, if necessary, the average of the four slopes is
calculated and divided by a predetermined factor, such as 16 for example,
to obtain a low slope threshold, Block 540. The low slope content is then
obtained by determining the number of sample points in the three-second
segment having an absolute slope less than or equal to the low slope
threshold, Block 542.

[0123]According to an embodiment of the present invention, if, during the
determination of the low slope threshold in Block 540, the low slope
threshold is a fraction, rather than a whole number, a correction is made
to the low slope content to add a corresponding fraction of the samples.
For example, if the threshold is determined to be 4.5, then the low slope
content is the number of sample points having an absolute slope less than
or equal to 4 plus one half of the number of sample points with slope
equal to 5.

[0124]The spectral width metric, which corresponds to an estimate of the
spectral width of the signal for the three-second segment associated with
each channel ECG1 and ECG2, is defined, for example, as the difference
between the mean frequency and the fundamental frequency of the signal.
According to an embodiment of the present invention, the spectral width
metric is calculated by determining the difference between the most
recent estimate of the RR-cycle length and the mean spectral period of
the signal for that channel. As is known in the art, the mean spectral
period is the inverse of the mean spectral frequency.

[0125]It is understood that R-R cycle length utilized in the concerned
state and armed state can be different than that used in the not
concerned state. For example, according to an embodiment of the present
invention, the 9th longest R-R interval is utilized in the not
concerned state and the mean of the 7th to the 10th R-R
interval is utilized in the concerned state and the armed state.

[0126]FIG. 10 is a graphical representation of a VF shock zone according
to an embodiment of the present invention. As illustrated in FIG. 10, a
VF shock zone 500 is defined for each channel ECG1 and ECG2 based on the
relationship between the calculated low slope content and the spectral
width associated with the channel. For example, the shock zone is defined
by a first boundary 502 associated with the low slope content set for by
the equation:

Low slope content=-0.0013×spectral width+0.415 Equation 1

and a second boundary 504 associated with the spectral width set forth by
the equation:

spectral width=200 Equation 2

[0127]As can be seen in FIG. 10, since noise 506 tends to have a
relatively higher spectral width, and normal sinus rhythm 508 tends to
have a relatively higher low slope content relative to VF, both noise 506
and normal sinus rhythm 508 would be located outside the VF shock zone
500.

[0128]A determination is made for each channel ECG1 and ECG2 as to whether
the low slope content for that channel is less than both the first
boundary 502 and the spectral width is less than the second boundary 504,
i.e., the low slope content is less than -0.0013×spectral
width+0.415, and the spectral width is less than 200. For example, once
the event is determined to be associated with VF, i.e., the intervals for
both channels are determined to be irregular, No in Block 356, a
determination is made that channel ECG1 is in the VF shock zone, Yes in
Block 360, if, for channel ECG1, both the low slope content is less than
the first boundary 502 and the spectral width is less than the second
boundary 504. The three second segment for that channel ECG1 is then
determined to be shockable, Block 363 and the associated buffer for that
channel is updated accordingly. If either the low slope content for the
channel is not less than the first boundary 502 or the spectral width is
not less than the second boundary, the channel ECG1 is determined not to
be in the VF shock zone, No in Block 360, the three second segment for
that channel ECG1 is then determined to be not shockable, Block 365, and
the associated buffer is updated accordingly.

[0129]Similarly, a determination is made that channel ECG2 is in the VF
shock zone, Yes in Block 362, if, for channel ECG2, both the low slope
content is less than the first boundary 502 and the spectral width is
less than the second boundary 504. The three second segment for that
channel ECG2 is then determined to be shockable, Block 369 and the
associated buffer for that channel is updated accordingly. If either the
low slope content for the channel is not less than the first boundary 502
or the spectral width is not less than the second boundary, the channel
ECG2 is determined not to be in the VF shock zone, No in Block 362, the
three second segment for that channel ECG2 is then determined to be not
shockable, Block 367, and the associated buffer is updated accordingly.

[0130]According to an embodiment of the present invention, rather than
being defined by Equation 1, the shock zone may be defined so that the
first boundary 502 associated with the low slope content is set forth by
the following equation:

Low slope content=-0.005×spectral width+1.1 Equation 1A

so that the VF shock zone 500 is defined, as described above, using
Equation 1A and Equation 2.

[0131]FIGS. 11A and 11B are graphical representations of the determination
of whether an event is within a shock zone according to an embodiment of
the present invention. During the determination of whether the event is
within the VT shock zone, Block 358 of FIG. 7B, the low slope content and
the spectral width is determined for each channel ECG1 and ECG2, as
described above in reference to determining the VF shock zone. A
determination is made as to which channel of the two signal channels ECG1
and ECG2 contains the minimum low slope content and which channel of the
two signal channels ECG 1 and ECG2 contains the minimum spectral width. A
first VT shock zone 520 is defined based on the relationship between the
low slope content associated with the channel determined to have the
minimum low slope content and the spectral width associated with the
channel determined to have the minimum spectral width. For example,
according to an embodiment of the present invention, the first VT shock
zone 520 is defined by a boundary 522 associated with the minimum low
slope content and the minimum spectral width set forth by the equation:

LSC=-0.004×SW+0.93 Equation 3

[0132]A second VT shock zone 524 is defined based on the relationship
between the low slope content associated with the channel determined to
have the minimum low slope content and the normalized mean rectified
amplitude associated with the channel determined to have the maximum
normalized mean rectified amplitude. The normalized mean rectified
amplitudes for the two channels ECG1 and ECG2 utilized during the VT
shock zone test is the same as described above in reference to the noise
determination of Block 343. For example, according to an embodiment of
the present invention, the second VT shock zone 524 is defined by a
second boundary 526 associated with the relationship between the minimum
low slope count and the maximum normalized mean rectified amplitude set
forth by the equation:

NMRA=68×LSC+8.16 Equation 4

[0133]If both the minimum low slope count is less than the first boundary
522, i.e., -0.004×minimum spectral width+0.93, and the maximum
normalized mean rectified amplitude is greater than the second boundary
526, i.e., 68×minimum low slope count+8.16, the event is determined
to be in the VT shock zone, YES in Block 358, and both channels ECG1 and
ECG2 are determined to be shockable, Block 357, and the associated
buffers are updated accordingly. If either the minimum low slope count is
not less than the first boundary 522 or the maximum normalized mean
rectified amplitude is not greater than the second boundary 526, the
event is determined to be outside the VT shock zone, NO in Block 358, and
both channels ECG1 and ECG2 are determined to be not shockable, Block
359.

[0134]As described, during both the VF shock zone test, Blocks 360 and
362, and the VT shock zone test, Block 358, the test results for each
channel ECG1 and ECG2 as being classified as shockable or not shockable
are stored in a rolling buffer containing the most recent eight such
designations, for example, for each of the two channels ECG1 and ECG2
that is utilized in the determination of Block 356, as described below.

[0135]If only one of the two channels ECG1 and ECG2 is determined to be
corrupted by noise, Yes in Block 346, a determination is made whether the
signal for the channel not corrupted by noise, i.e., the "clean channel",
is more likely associated with a VT event or with a VF event by
determining whether the signal for the clean channel includes R-R
intervals that are regular and can be therefore classified as being
relatively stable, Block 348. If the R-R intervals are determined not to
be relatively stable, NO in Block 348, the signal for the clean channel
is identified as likely being associated with VF, which is then verified
by determining whether the signal for the clean channel is in a VF shock
zone, Block 350, described below. If R-R intervals for the clean channel
are determined to be stable, YES in Block 348, the signal is identified
as likely being associated with VT, which is then verified by determining
whether the signal for the clean channel is in a VT shock zone, Block
352.

[0136]According to an embodiment of the present invention, in order to
determine whether the signal for the clean channel includes R-R intervals
that are regular and the clean channel can be therefore classified as
being either relatively stable, Yes in Block 348, or relatively unstable,
No in Block 348, the device discriminates VT events from VF events in
Block 348 by determining whether the relative level of variation in the
RR-intervals associated with the clean channel is regular. For example,
as illustrated in FIG. 7H, predetermined maximum and minimum intervals
for the clean channel are identified, Block 700, from the updated buffer
of 12 RR-intervals, Block 342 of FIG. 7B. According to an embodiment of
the present invention, the largest RR-interval and the sixth largest
RR-interval of the twelve RR-intervals are utilized as the maximum
interval and the minimum interval, respectively.

[0137]The difference between the maximum RR-interval and the minimum
RR-interval of the 12 RR-intervals is calculated to generate an interval
difference associated with the clean channel, 702. A determination is
then made as to whether the interval difference is greater than a
predetermined stability threshold, Block 704, such as 110 milliseconds,
for example.

[0138]If the interval difference is greater than the stability threshold,
the event is classified as an unstable event, Block 706, and therefore
the clean channel is determined not to include regular intervals, No in
Block 348, and a determination is made as to whether the signal
associated with the clean channel is within a predetermined VF shock
zone, Block 350 of FIG. 7B, described below. If the interval difference
is less than or equal to the stability threshold, No in Block 704, the
device determines whether the minimum RR interval is greater than a
minimum interval threshold, Block 710, such as 200 milliseconds, for
example.

[0139]If the minimum interval is less than or equal to the minimum
interval threshold, No in Block 710, the event is classified as an
unstable event, Block 706, and therefore the clean channel is determined
not to include regular intervals, No in Block 348, and a determination is
made as to whether the signal associated with the clean channel is within
a predetermined VF shock zone, Block 350 of FIG. 7B, described below. If
the minimum interval is greater than the minimum interval threshold, Yes
in Block 710, the device determines whether the maximum interval is less
than or equal to a maximum interval threshold, Block 712, such as 333
milliseconds for example. If the maximum interval is greater than the
maximum interval threshold, the event is classified as an unstable event,
Block 706, and therefore the clean channel is determined not to include
regular intervals, No in Block 348, and a determination is made as to
whether the signal associated with the clean channel is within a
predetermined VF shock zone, Block 350 of FIG. 7B, described below. If
the maximum interval is less than or equal to the maximum interval
threshold, the event is classified as a stable event, Block 714, and
therefore the clean channel is determined to include regular intervals,
Yes in Block 348, and a determination is made as to whether the signal
associated with the clean channel is within a predetermined VT shock
zone, Block 352 of FIG. 7B, described below.

[0140]Returning to FIG. 7B, according to an embodiment of the present
invention, the determination of whether the clean channel is within the
VF shock zone, Block 350, is made based upon a low slope content metric
and a spectral width metric, similar to the VF shock zone determination
described above in reference to Blocks 360 and 362, both of which are
determined for the clean channel using the method described above. Once
the low slope content metric and a spectral width metric are determined
for the clean channel, the determination of whether the clean channel is
in the VF shock zone is made using Equations 1 and 2, so that if either
the low slope content for the clean channel is not less than the first
boundary 502 or the spectral width is not less than the second boundary
504, the clean channel is determined not to be in the VF zone, No in
Block 350 and both channels are classified as not shockable, Block 351,
and the associated buffers are updated accordingly.

[0141]If the low slope content for the clean channel is less than the
first boundary 502 and the spectral width is less than the second
boundary 504, the clean channel is determined to be in the VF zone, Yes
in Block 350. A determination is then made as to whether the channel
determined to be corrupted by noise, i.e., the "noisy channel", is within
the VF shock zone, Block 354. If either the low slope content for the
noisy channel is not less than the first boundary 502 or the spectral
width is not less than the second boundary 504, the noisy channel is
determined not to be in the VF zone, No in Block 354, the clean channel
is classified as shockable and the noisy channel is classified as not
shockable, Block 355, and the associated buffers are updated accordingly.

[0142]If the low slope content for the noisy channel is less than the
first boundary 502 and the spectral width is less than the second
boundary 504, the noisy channel is determined to be in the VF zone, Yes
in Block 354, both the clean channel and the noisy channel are classified
as being shockable, Block 353, and the associated buffers are updated
accordingly.

[0143]Similar to the VT shock zone determination described above in
reference to Block 358, during the determination as to whether the clean
channel is within the VT shock zone in Block 352, the low slope content
and the spectral width is determined for the clean channel as described
above in reference to determining the VF shock zone. The first VT shock
zone 520 is defined based on the relationship between the low slope
content and the spectral width associated with the clean channel
according to Equation 3, for example, and the second VT shock zone 524 is
defined based on the relationship between the low slope count and the
normalized mean rectified amplitude associated with the clean channel.
The normalized mean rectified amplitudes for the clean channel is the
same as described above in reference to the noise detection tests of
Block 344. For example, according to an embodiment of the present
invention, the second VT shock zone 524 is defined by a second boundary
526 associated with the relationship between the low slope count and the
normalized mean rectified amplitude of the clean channel using Equation
4.

[0144]If both the low slope count is less than the first boundary 522,
i.e., -0.004×spectral width of clean channel+0.93, and the
normalized mean rectified amplitude is greater than the second boundary
526, i.e., 68×low slope count of clean channel+8.16, the clean
channel is determined to be in the VT shock zone, Yes in Block 352, both
channels are classified as being shockable, Block 353, and the associated
buffers are updated accordingly.

[0145]If either the low slope count is not less than the first boundary
522 or the maximum normalized mean rectified amplitude is not greater
than the second boundary 526, the clean channel is determined to be
outside the VT shock zone, No in Block 352, both channels are classified
as being not shockable, Block Block 351, and the associated buffers are
updated accordingly.

[0146]FIG. 12 is a graphical representation of a shock zone according to
an embodiment of the present invention. According to an embodiment of the
present invention, during the determination of whether the signal
associated with each of the channels ECG1 and ECG2 is within the VF shock
zone, Blocks 360 and 362, the VF shock zone is defined based upon a
normalized mean rectified amplitude metric and a spectral width metric
for each of the two channels ECG1 and ECG2, both of which may be
generated, for example, as described above. In particular, according to
the embodiment of FIG. 12, a VF shock zone 800 is defined for each
channel ECG1 and ECG2, during a given three second sensing window, based
on the relationship between the calculated normalized mean rectified
amplitude and the spectral width associated with the channel, with the
shock zone 800 being defined by a boundary 802 associated with the
normalized mean rectified amplitude by the equation:

Normalized Mean Rectified Amplitude=0.2×spectral width+3 Equation.
5

[0147]As can be seen in FIG. 12, since noise 806 tends to have a
relatively higher spectral width, and normal sinus rhythm 808 tends to
have a relatively lower normalized mean rectified amplitude relative to
VF, both noise 806 and normal sinus rhythm 808 are located outside the VF
shock zone 800. Therefore, a determination is made for each channel ECG1
and ECG2, during a given three second sensing window, as to whether the
normalized mean rectified amplitude for that channel is greater than or
equal to the boundary 802, i.e., the normalized mean rectified amplitude
is greater than or equal to 0.2×spectral width+3. For example, once
the event is determined to be associated with VF, i.e., the intervals for
both channels are determined to be irregular, No in Block 356, a
determination is made that channel ECG1 is in the VF shock zone, Yes in
Block 360, if, for channel ECG1, the normalized mean rectified amplitude
is greater than or equal to the boundary 802. The three second segment
for that channel ECG1 is then determined to be shockable, Block 363 and
the associated buffer for that channel is updated accordingly. If the
normalized mean rectified amplitude for the channel is less than the
boundary 802, the channel ECG1 is determined not to be in the VF shock
zone, No in Block 360, the three second segment for that channel ECG1 is
then determined to be not shockable, Block 365, and the associated buffer
is updated accordingly.

[0148]Similarly, a determination is made that channel ECG2 is in the VF
shock zone, Yes in Block 362, if, for channel ECG2, the normalized mean
rectified amplitude is greater than or equal to the boundary 802. The
three second segment for that channel ECG2 is then determined to be
shockable, Block 369 and the associated buffer for that channel is
updated accordingly. If the normalized mean rectified amplitude for the
channel is less than the boundary 802, the channel ECG2 is determined not
to be in the VF shock zone, No in Block 362, the three second segment for
that channel ECG2 is then determined to be not shockable, Block 367, and
the associated buffer is updated accordingly.

[0149]FIG. 13 is a graphical representation of the determination of
whether an event is within a shock zone according to an embodiment of the
present invention. According to another embodiment of the present
invention, during the determination of whether the event is within the VT
shock zone, Block 358 of FIG. 7B, the normalized mean rectified amplitude
and the spectral width are determined for each channel ECG1 and ECG2
during a given three second sensing window, as described above in
reference to determining the VF shock zone. A determination is made as to
which channel of the two signal channels ECG1 and ECG2 contains the
maximum normalized mean rectified amplitude and which channel of the two
signal channels ECG 1 and ECG2 contains the minimum spectral width. A VT
shock zone 820 is defined based on the relationship between the
normalized mean rectified amplitude associated with the channel
determined to have the maximum normalized mean rectified amplitude and
the spectral width associated with the channel determined to have the
minimum spectral width. For example, according to an embodiment of the
present invention, the VT shock zone 820 is defined, for a given three
second sensing window, by a boundary 822 associated with the maximum
normalized mean rectified amplitude and the minimum spectral width set
forth by the equation:

NMRA=0.3636×SW-15 Equation 6

[0150]If the maximum normalized mean rectified amplitude is greater than
or equal to the boundary 822, i.e., 0.3636×minimum spectral
width-15, the event is determined to be in the VT shock zone, YES in
Block 358, and the three second segment for both channels ECG1 and ECG2
are determined to be shockable, Block 357, and the associated buffers are
updated accordingly. If the maximum normalized mean rectified amplitude
is less than the boundary 822, the event is determined to be outside the
VT shock zone, NO in Block 358, and both channels ECG1 and ECG2 are
determined to be not shockable, Block 359.

[0151]Returning to FIG. 7B, according to another embodiment of the present
invention, the determination of whether the clean channel is within the
VF shock zone, Block 350, is made based upon a normalized mean rectified
amplitude metric and a spectral width metric, similar to the VF shock
zone determination described above in reference to Blocks 360 and 362,
both of which are determined for the clean channel using the VF shock
zone described above in reference to FIG. 12. In particular, once the
normalized mean rectified amplitude metric and a spectral width metric
are determined for the clean channel, the determination of whether the
clean channel is in the VF shock zone is made using Equation 5, so that
if the normalized mean rectified amplitude for the clean channel is less
than the boundary 802, the clean channel is determined not to be in the
VF zone, No in Block 350 and both channels are classified as not
shockable, Block 351, and the associated buffers are updated accordingly.

[0152]If the normalized mean rectified amplitude for the clean channel is
greater than or equal to the boundary 802, the clean channel is
determined to be in the VF zone, Yes in Block 350. A determination is
then made as to whether the channel determined to be corrupted by noise,
i.e., the "noisy channel", is within the VF shock zone, Block 354. If the
normalized mean rectified amplitude for the noisy channel is less than
the boundary 802, the noisy channel is determined not to be in the VF
zone, No in Block 354, the clean channel is classified as shockable and
the noisy channel is classified as not shockable, Block 355, and the
associated buffers are updated accordingly.

[0153]If the normalized mean rectified amplitude for the noisy channel is
greater than or equal to the boundary 802, the noisy channel is
determined to be in the VF zone, Yes in Block 354, both the clean channel
and the noisy channel are classified as being shockable, Block 353, and
the associated buffers are updated accordingly.

[0154]Similar to the VT shock zone determination described above in Block
358 using the VT shock zone of FIG. 13, during the determination as to
whether the clean channel is within the VT shock zone in Block 352, the
normalized mean rectified amplitude and the spectral width are determined
for the clean channel as described above in reference to determining the
VF shock zone. The VT shock zone 820 is then defined based on the
relationship between the normalized mean rectified amplitude and the
spectral width associated with the clean channel according to Equation 6.
If the maximum normalized mean rectified amplitude for the clean channel
is greater than or equal to the boundary 822, i.e., 0.3636×minimum
spectral width-15, the clean channel is determined to be in the VT shock
zone, YES in Block 352, and both channels ECG1 and ECG2 are determined to
be shockable, Block 353, and the associated buffers are updated
accordingly. If the maximum normalized mean rectified amplitude for the
clean channel is less than the boundary 822, the clean channel is
determined to be outside the VT shock zone, NO in Block 352, and both
channels ECG1 and ECG2 are determined to be not shockable, Block 351.

[0155]Once the classification of both of the channels ECG1 and ECG2 is
made subsequent to the determination of whether the clean channel or
channels is in the VT shock zone, Block 352 and 358, or the VF shock
zone, Blocks 350 and Blocks 360 and 362 in combination, a determination
is made as to whether the device should transition from the concerned
state 304 to the armed state 306, Block 370. For example, according to an
embodiment of the present invention, the transition from the concerned
state 304 to the armed state 306 is confirmed if a predetermined number,
such as two out of three for example, of three-second segments for both
channels ECG1 and ECG2 have been classified as being shockable. If the
predetermined number of three-second segments in both channels ECG1 and
ECG2 have been classified as shockable, the device transitions from the
concerned state 304 to the armed state 306, Yes in Block 370. If the
predetermined number of three-second segments in both channels ECG1 and
ECG2 have not been classified as shockable, the device does not
transition from the concerned state 304 to the armed state 306, no in
Block 370, and a determination as to whether to transition back to the
not concerned state 302 is made, Block 372. The determination as to
whether to transition from the concerned state 304 back to the not
concerned state 302 is made, for example, by determining whether the
heart rate estimate is less than a heart rate threshold level in both of
the two channels ECG1 and ECG2. If it is determined that the device
should not transition to the not concerned state 302, i.e., both of the
two heart rate estimates are greater than the heart rate threshold, the
process is repeated using the signal generated during a next three-second
window, Block 341.

[0156]According to an embodiment of the present invention, the heart rate
threshold level is set as 180 bpm, for example, and a single estimate of
heart rate (that occurs every three seconds) in at least one of the two
channels ECG1 and ECG2 that is less than the heart rate threshold level
will suffice to cause the device to transition from the concerned state
304 to the not concerned state 302, Yes in Block 372.

[0157]When the device transitions from the concerned state 304 to the
armed state 306, Yes in Block 370, processing continues to be triggered
by a three-second time out as is utilized during the concerned state 304,
described above. As illustrated in FIG. 7F, once the device transitions
from the concerned state 302 to the armed state 306, charging of the
capacitors is initiated, Block 600. During the charging of the
capacitors, the classification of segments for each channel ECG1 and ECG2
as being either shockable or not shockable generated during the shock
zone tests described above continues and once the next three seconds of
data has been acquired, Block 601, a determination is made as whether the
event continues to be a shockable event by determining whether a
predetermined number of segments, such as the most recent two segments
for example, have been classified in both of the two channels ECG1 and
ECG2 as not shockable, Block 602. If the predetermined number of three
second segments have been classified as not shockable, indicating that
the event may possibly no longer be a shockable event, Yes in Block 602,
the charging of the capacitors is stopped, Block 604, and a determination
is made as to whether to transition to the not concerned state 302, Block
606.

[0158]According to an embodiment of the present invention, the device will
transition from the armed state 306 to the not concerned state 302, Yes
in Block 606, if certain termination requirements are met. For example,
return to the not concerned state 302 occurs if, for both channels ECG1
and ECG2 simultaneously, less than two out of the last three three-second
segments are classified as shockable, less than three out of the last
eight three-second segments are classified as shockable, and the most
recent three second segment is classified as not shockable. Another
possible criteria for returning to the not concerned state 302 is the
observation of 4 consecutive not shockable classifications in both
channel ECG1 and ECG2 simultaneously.

[0159]In addition to the two criteria described above, at least one of the
current heart rate estimates must be slower than the programmed rate
threshold 403, and capacitor charging must not in progress. If each of
these requirements are satisfied, Yes in Block 606, the device
transitions from the armed state 306 to the not concerned state 302.

[0160]If one or more of these requirements are determined not to be
satisfied, return to the not concerned state is not indicated, No in
Block 606, and a determination is then made as whether the shockable
rhythm is redetected, Block 608, by determining whether predetermined
redetection requirements have been satisfied. For example, a
determination is made as to whether a predetermined number of
three-second segments in both of the two channels ECG1 and ECG2, such as
two out of the most recent three for example, have been classified as
being shockable. If the predetermined redetection requirements are not
satisfied, No in Block 608, the determination of whether to terminate
delivery of the therapy, Block 606, is repeated so that the processing
switches between the determination of whether to terminate delivery of
therapy, Block 606 and the determination as to whether the shockable
event is redetected, Block 608, until either the event has terminated and
the device transitions from the armed state 306 to the not concerned
state 302 or the event is redetected. If the predetermined redetection
requirements are met, Yes in Block 608, charging is re-initiated, Block
600, and the process is repeated.

[0161]If, during the charging of the capacitors, the predetermined number
of three second segments have not been classified as not shockable, No in
Block 602, a determination is made as to whether the charging of the
capacitors is completed, Block 610. As long as the predetermined number
of three second segments continue to be classified as shockable, No in
Block 602, charging of the capacitors continues until charging is
completed. Once the charging of the capacitors is completed, Yes in Block
610, a determination is made as to whether delivery of the therapy is
still appropriate, Block 612, by determining whether predetermined
therapy delivery confirmation requirements have been satisfied. For
example, according to an embodiment of the present invention, the
predetermined therapy delivery confirmation requirements include
determining whether, for both channels ECG1 and ECG2, at least five out
of the last eight three-second segments are classified as being
shockable, and at least two of the last three three-second segments are
classified as being shockable. In addition, a determination is made as to
whether the most recent three-second segment has been classified as being
shockable for at least one of the two channels ECG1 and ECG2.

[0162]If the predetermined therapy delivery requirements have not been
satisfied, and therefore the delivery of the therapy is not confirmed, No
in Block 612, the determination of whether to transition from the armed
state 306 to the not concerned state 302, Block 606, is repeated. If the
predetermined therapy delivery requirements are satisfied, and therefore
the delivery of the therapy is confirmed, Yes in Block 612, the device
transitions from the armed state 306 to the shock state 308.

[0163]As illustrated in FIG. 7G, once the device transitions from the
armed state 306 to the shock state 308, the therapy is delivered upon
observation of the first sensed R-wave, Block 630, the episode data is
stored, Block 632, and the buffers for storing the eight three second
segments are cleared, Block 634. Once a post shock timer, such as three
seconds for example, has expired, Yes in Block 636, the device
transitions from the shock state 308 to Block 606 of the armed state 306.
Since, as described above, classification of at least three subsequent
three-second segments is required before the termination decision can be
made in Block 606 subsequent to the delivery of therapy in the shock
state 308, a determination based on the termination requirements cannot
be initiated until at least twelve seconds after the initial shock
therapy was delivered. The termination and redetection requirements are
then reviewed until one of the two requirements are satisfied, i.e., the
event is determined to have terminated, Yes in Block 606, or the event is
redetected, Yes in Block 608. If the redetection requirements are
satisfied, the charging of the capacitors is again initiated, Block 600,
and processing in the armed state 306 continues as described above until
all available therapies have been exhausted.

[0164]FIGS. 14A-14C are graphical representations illustrating the
occurrence of oversensing due to a slow monomorphic ventricular
tachycardia with a wide QRS complex. The graphical representation in FIG.
14A is an exemplary illustration of a slow (less than the VT/VF
threshold, i.e., 180 bpm) monomorphic ventricular tachycardia with a wide
QRS complex, commonly referred to as a slow VT, sensed via the first
sensing channel ECG1 and the second sensing channel ECG2. A resulting
filtered and rectified signal 804 for the first sensing channel ECG1 is
shown in FIG. 14B and a resulting filtered and rectified signal 806 for
the second sensing channel ECG2 is shown in FIG. 14C. As can be seen in
FIGS. 14A-14C, oversensing tends to occur during slow VT because each QRS
complex results in two peak amplitudes 808 and 810 that exceed a sensing
threshold 812 in one or more of the sensing channels ECG1 and ECG2,
resulting in double counting of R-waves in one or more of the sensing
channels ECG1 and ECG2. As a result, although the actual true heart rate
occurring during slow VT may be less than the VT/VF threshold (i.e., 180
bpm), this oversensing resulting from the double counting of R-waves
during such events causes the rhythm to be erroneously identified as a
VT/VF rhythm.

[0165]FIG. 15 is a flowchart of a method for detecting cardiac events in a
medical device according to an embodiment of the present invention. The
method illustrated in FIG. 15 is similar to the method described above in
reference to FIG. 7A, but differs in that according to the method for
detecting cardiac events according to the embodiment of FIG. 15 includes
the additional process of detecting oversensing, which is described
below. The process leading up to the oversensing detection is the same as
described in FIG. 7A and will not be repeated here for brevity sake.

[0166]As illustrated in FIG. 15, once a VT/VF event has been determined to
be present, i.e., the final heart rate estimates for both channels are
determined to be greater than the predetermined VT/VF threshold, Yes in
Block 336, a determination is made as to whether this determination
occurred as a result of oversensing, Block 820. If oversensing is
determined to have occurred, Yes in Block 820, the determined heart rate
is corrected for the oversensing, Block 822, and a determination is made
as to whether the corrected heart rate is greater than the predetermined
VT/VF threshold, Block 824. If the corrected heart rate is not greater
than the predetermined VT/VF threshold, No in Block 824, the buffer
containing the 12 R-R intervals for the channel where the corrected heart
rate not greater than the predetermined VT/VF threshold is updated by
removing the first R-sense, shifting the remaining eleven R-sense samples
back so that the second R-sense becomes the first R-sense, and so forth,
and inserting the next detected R-sense, Block 322, as the twelfth
R-sense. A new current heart rate estimate is then determined, Block 323
and the process is repeated.

[0167]If oversensing is not determined to have occurred, No in Block 820,
or if oversensing is determined to be present and the subsequently
generated corrected heart rate Block 822 is determined to be greater than
the VT/VF threshold, Yes in Block 824, a VT/VF event is determined to be
present and the process transitions from the not concerned state 302 to
the concerned state 304.

[0168]FIGS. 16A and 16B are flowcharts of a method of determining whether
oversensing has occurred according to an embodiment of the present
invention. According to an embodiment of the present invention, during
the determination in Block 820 of whether oversensing has occurred, a
determination is made as to whether predetermined criteria associated
with oversensing are met. For example, as illustrated in FIG. 16A, if,
after determining that both heart rate estimates are greater than VT/VF
threshold, Block 336 of FIG. 15, both channels ECG1 and ECG2 were
determined to be reliable, a spectral width is determined for each
channel ECG1 and ECG2, Block 830, and a determination is made as to which
one of the spectral width for the first channel ECG1 and the spectral
width of the second channel ECG2 is the maximum spectral width, Block
832. A determination is then made as to whether the maximum spectral
width is less than or equal to a spectral width threshold, Block 834.
According to an embodiment of the present invention, the spectral width
threshold is set as -20, although it is understood that any desired value
may be utilized.

[0169]If the maximum spectral width is not less than or equal to the
spectral width threshold, No in Block 834, oversensing is determined not
to be occurring, and the device transitions to the concerned state, Block
304. If the maximum spectral width is less than or equal to the spectral
width threshold, Yes in Block 834, a metric of signal energy content is
determined for each channel ECG1 and ECG2, Block 836, and a determination
is made as to which one of the metric of signal energy content for the
first channel ECG1 and the metric of signal energy content for the second
channel ECG2 is the minimum metric of signal energy content, Block 838. A
determination is then made as to whether the minimum metric of signal
energy content is greater than a metric of signal energy content
threshold, Block 840. According to an embodiment of the present
invention, the metric of signal energy content is a normalized mean
rectified amplitude, generated as described above, and the metric of
signal energy content threshold is 40.

[0170]If the minimum metric of signal energy content is not greater than
the metric of signal energy content threshold, No in Block 840,
oversensing is determined to not be occurring, and the device transitions
from the not concerned state 302 to the concerned state 304. If the
minimum metric of signal energy content is greater than the metric of
signal energy content threshold, Yes in Block 840, a heart rate metric
difference is determined for each channel ECG1 and ECG2, Block 842, and a
determination as to which one of the heart rate metric difference for the
first channel ECG1 and the heart rate metric difference for the second
channel ECG2 is the maximum heart rate metric difference, Block 844. A
determination is then made as to whether the maximum heart rate metric
difference is less than or equal to a heart rate metric difference
threshold, Block 846, such as 52.5 ms for example. If the maximum heart
rate metric difference is greater than the heart rate metric difference
threshold, No in Block 846, oversensing is determined to not be
occurring, and the device transitions from the not concerned state 302 to
the concerned state 304. If the maximum heart rate metric difference is
less than or equal to the heart rate metric difference threshold, Yes in
Block 846, oversensing is determined to likely be occurring and the
corrected rate is determined, Block 822 of FIG. 15, described below.

[0171]According to an embodiment of the present invention, the heart rate
metric difference is derived from the 12 RR intervals currently stored in
the buffer so that the heart rate metric difference for each channel ECG
1 and ECG2 is generated by first determining the trimmed mean of the
third through the tenth RR intervals TM =Mean {RR3: RR10}, and
calculating an absolute difference between the second RR interval and the
trimmed mean |RR2-TM|, between the fifth RR interval and the trimmed
mean |RR5-TM|, between the eight RR interval and the trimmed mean
|RR8-TM|, and between the eleventh RR interval and the trimmed mean
|RR11-TM|. The heart rate metric difference for the channel is then
set equal to the average of the four calculated absolute differences,

[0172]As illustrated in FIG. 16B, if, after determining that both heart
rate estimates are greater than the VT/VF threshold, Block 336, only one
of the channels ECG1 and ECG2 was determined to be reliable, No in Block
328 and Yes in Block 330, the oversensing criteria are applied using only
the reliable channel. For example, if the first channel ECG1 was
determined to be unreliable and the second channel ECG2 was determined to
be the reliable channel, a spectral width is determined for the reliable
channel ECG2, Block 850, and a determination is made as to whether the
spectral width is less than or equal to the spectral width threshold,
Block 852.

[0173]If the spectral width is not less than or equal to the spectral
width threshold, No in Block 852, oversensing is determined not to be
occurring, and the device transitions to the concerned state, Block 304.
If the spectral width is less than or equal to the spectral width
threshold, Yes in Block 852, a metric of signal energy content is
determined for the reliable channel ECG2, Block 854, and a determination
is made as to whether the metric of signal energy content is greater than
the metric of signal energy content threshold, Block 856.

[0174]If the metric of signal energy content is not greater than the
metric of signal energy content threshold, No in Block 856, oversensing
is determined to not be occurring, and the device transitions from the
not concerned state 302 to the concerned state 304. If the metric of
signal energy content is greater than the metric of signal energy content
threshold, Yes in Block 856, a heart rate metric difference is determined
for the reliable channel ECG2, Block 858, and a determination as to
whether the heart rate metric difference is less than or equal to the
heart rate metric difference threshold, Block 860. If the heart rate
metric difference is greater than the heart rate metric difference
threshold, No in Block 860, oversensing is determined to not be
occurring, and the device transitions from the not concerned state 302 to
the concerned state 304. If the heart rate metric difference is less than
or equal to the heart rate metric difference threshold, Yes in Block 860,
oversensing is determined to likely be occurring and the corrected rate
is determined, Block 822 of FIG. 15, using the rate correction technique
described below.

[0175]It is understood that while the method of determining whether
oversensing has occurred described above in reference to FIGS. 16A and
16B includes three oversensing characteristics that must be satisfied,
spectral width, the metric of signal energy content, and the heart rate
metric difference, in order for oversensing to be detected, the present
invention may require that a combination of any two, or only one of the
three oversensing characteristics to be satisfied in order for
oversensing to be detected. In addition, while the three oversensing
characteristics are described in terms of the spectral width being the
first to be determined, followed by the metric of signal energy content,
and then the heart rate metric difference, if more than one of the
oversensing characteristics is utilized, they may occur in any order.

[0176]FIG. 17 is a flowchart of a method of determining whether
oversensing has occurred according to an embodiment of the present
invention. As illustrated in FIGS. 7B and 17, once the device has
transitioned from the not concerned state 302 to the concerned state 304,
and it is determined that noise was detected in only one of the channels,
Yes in Block 346, a determination is made as to whether oversensing has
occurred in the clean channel, Block 862. Since the oversensing is
determined for only one of the two channels ECG1 and ECG 2, i.e., the
non-noisy or clean channel, the oversensing criteria as describe above in
reference to FIG. 16B is utilized in the oversensing determination of
Block 862. If oversensing is not detected, No in Block 862, the
above-described determination of whether there are regular intervals in
the clean channel, Block 348, is made and the process continues in FIG.
7B as described above.

[0177]If oversensing is determined to be detected, a corrected rate for
the clean channel is determined, Block 863, using the rate correction
process described below, and a determination is then made as to whether
the corrected rate is greater than the VT/VF detection threshold, Block
864. If the corrected heart rate is greater than the VT/VF threshold, the
above-described determination of whether there are regular intervals in
the clean channel, Block 348, is made and the process continues in FIG.
7B as described above. If the corrected rate is not greater than the
VT/VF threshold, No in Block 864, the clean channel is classified as
being unshockable, Block 865, and the determination is made as to whether
the device should transition from the concerned state 304 to the armed
state 306, Block 370, as described above.

[0178]If it is determined that both channels ECG1 and ECG2 are noise-free,
i.e., both are clean channels, No in Block 346, a determination is made
as to whether oversensing has occurred, Block 866, using both channels
ECG1 and ECG2. Since the oversensing is determined using both channels
ECG1 and ECG 2, the oversensing criteria as describe above in reference
to FIG. 16A is utilized in the oversensing determination of Block 866. If
oversensing is not detected, No in Block 866, the determination of
whether there are regular intervals in both channels ECG1 and ECG2, Block
356, is made and the process continues in FIG. 7B as described above.

[0179]If oversensing is determined to be detected, a corrected rate is
determined for each channel ECG1 and ECG2, Block 867, using the rate
correction process described below, and a determination is then made as
to whether the corrected rates for both channels ECG 1 and ECG2 are
greater than the VT/VF detection threshold, Block 868. If the corrected
heart rates for both channels ECG1 and ECG2 are greater than the VT/VF
threshold, Yes in Block 868, the determination of whether there are
regular intervals in both channels, Block 356, is made and the process
continues in FIG. 7B as described above. If the corrected rates for both
channels ECG1 and ECG2 are not greater than the VT/VF threshold, No in
Block 868, a determination is made as to whether the corrected rate for
both of the channels ECG1 and ECG2 is less than or equal to the VF/VF
threshold, Block 869. If the corrected rates for both of the channels
ECG1 and ECG2 is less than or equal to the VF/VF threshold, Yes in Block
869, i.e., both channels ECG1 and ECG2 are slow, and therefore both
channels ECG1 and ECG 2 are classified as being not shockable, Block 870,
and the determination is made as to whether the device should transition
from the concerned state 304 to the armed state 306, Block 370, as
described above.

[0180]If the corrected rates for both of the channels ECG1 and ECG2 are
not less than or equal to the VF/VF threshold, No in Block 869,
indicating that the corrected rate for one of the channels ECG1 and ECG2
is less than or equal to the VF/VF threshold and the corrected rate for
the other of the channels ECG1 and ECG2 is greater than the VT/VF
threshold, the channel that was determined to be less than the VT/VF
threshold, i.e., the slow channel, is classified as being non-shockable,
Block 871, and a determination is made as to whether there are regular
intervals in the channel that was greater than the VT/VF threshold, i.e.,
the fast channel, Block 872, using the regular intervals processing as
described above, indicating the channel can be classified as being
stable.

[0181]If the fast channel is determined to have regular intervals, a
determination is made as to whether the fast channel is in the VT shock
zone, Block 873, using the VT shock zone determination described above.
If the fast channel is in the VT shock zone, the fast channel is
classified as shockable, Block 874, and the determination is made as to
whether the device should transition from the concerned state 304 to the
armed state 306, Block 370, as described above. If the fast channel is
not in the VT zone, the fast channel is classified as being not
shockable, Block 875, and the determination is made as to whether the
device should transition from the concerned state 304 to the armed state
306, Block 370, as described above.

[0182]If the fast channel is determined not to have regular intervals, No
in Block 872, a determination is made as to whether the fast channel is
in the VF shock zone, Block 876, using the VF shock zone criteria
described above. If the fast channel is in the VF shock zone, the fast
channel is classified as shockable, Block 874, and the determination is
made as to whether the device should transition from the concerned state
304 to the armed state 306, Block 370, as described above. If the fast
channel is not in the VF zone, the fast channel is classified as being
not shockable, Block 875, and the determination is made as to whether the
device should transition from the concerned state 304 to the armed state
306, Block 370, as described above.

[0183]FIG. 18 is a flowchart of a method of determining whether
oversensing has occurred according to an embodiment of the present
invention. As illustrated in FIG. 18, in order to determine whether the
fast channel has regular intervals, Block 872 of FIG. 17, and can
therefore be classified as being relatively stable, predetermined maximum
and minimum intervals for the fast channel are identified, Block 877,
using the updated buffer of 12 RR-intervals. According to one embodiment
of the present invention, the largest RR-interval and the sixth largest
RR-interval of the twelve RR-intervals are utilized as the maximum
interval and the minimum interval, respectively.

[0184]The difference between the maximum RR-interval and the minimum
RR-interval of the 12 RR-intervals is calculated, Block 878, to generate
an interval difference associated with the fast channel. A determination
is made as to whether the interval difference is greater than a
predetermined stability threshold, Block 879, such as 110 milliseconds,
for example.

[0185]If the interval difference is greater than the stability threshold,
the segment is classified as an unstable segment, Block 880, and a
determination is made as to whether the signal associated with the fast
channel is within a predetermined VF shock zone, Block 876, described
above. If the interval difference is less than or equal to the stability
threshold, No in Block 879, the device determines whether the minimum
RR-interval associated with the fast channel is greater than a minimum
interval threshold, Block 881, such as 200 milliseconds, for example.

[0186]If the minimum RR interval is less than or equal to the minimum
interval threshold, the segment is classified as an unstable segment,
Block 880, and a determination is made as to whether the signal
associated with the fast channel is within a predetermined VF shock zone,
Block 876, described above. If the minimum interval is greater than the
minimum interval threshold, the device determines whether the maximum
RR-interval associated with the fast channel is less than or equal to a
maximum interval threshold, Block 882, such as 333 milliseconds for
example. If the maximum interval is greater than the maximum interval
threshold, the segment is classified as an unstable segment, Block 880,
and a determination is made as to whether the signal associated with the
fast channel is within a predetermined VF shock zone, Block 876,
described above.

[0187]If the maximum interval is less than or equal to the maximum
interval threshold, the segment is classified as a stable segment, Block
883, and a determination is made as to whether the signal associated with
the fast channel is within a predetermined VT shock zone, Block 873,
described above.

[0188]It should be noted that, as described above in reference to FIG. 7F,
when the device transitions from the concerned state 304 to the armed
state 306, Yes in Block 370, processing continues to be triggered by a
three-second time out as is utilized during the concerned state 304,
described above, and once the device transitions from the concerned state
302 to the armed state 306, charging of the capacitors is initiated,
Block 600. During the charging of the capacitors, the classification of
segments for each channel ECG1 and ECG2 as being either shockable or not
shockable generated during the shock zone tests described above
continues, and, according to an embodiment of the present invention, this
classification of the segments while in the armed state 306 includes the
determination of whether oversensing has occurred, along with the
generation of the corrected rate when oversensing has occurred, as
described above.

[0189]FIGS. 19A and 19B are graphical representations of determining a
corrected heart rate in response to oversensing according to an
embodiment of the present invention. FIGS. 20A and 20B are flowcharts of
a method of determining a corrected heart rate in response to oversensing
according to an embodiment of the present invention. As illustrated in
FIGS. 19A, 19B and FIGS. 20A and 20B, during the determination of an
updated or corrected heart rate in response to determining the presence
of oversensing, the device first determines a maximum amplitude 886 and
an absolute value of the minimum amplitude for the current three second
window, Block 900, and determines whether the maximum amplitude for the
three second window is greater than the absolute value of the minimum
amplitude for the three second window, Block 901. If the maximum
amplitude for the three second window is not greater than the absolute
value of the minimum amplitude for the three second window, No in Block
901, the device uses the minimum amplitudes to determine the corrected
rates, Block A, described below in FIG. 20B.

[0190]In order to determine an updated or corrected heart rate, a
determination is made as to whether certain predetermined criteria
associated with the corrected heart rate are met. For example, if the
maximum amplitude 886 for the three second window is greater than the
absolute value of the minimum amplitude for the three second window, Yes
in Block 901, the device divides the three second window 884 into a
predetermined number of window sub-segments 885, Block 902. A local
maximum amplitude 887 is determined for one of the sub-segments 885,
Block 904, and a determination is made as to whether the local maximum
887 for that sub-segment 885 is greater than a predetermined local
maximum amplitude threshold, Block 906. According to an embodiment of the
present invention, the predetermined local maximum amplitude threshold is
a percentage of the overall maximum amplitude 886 for the sensing window,
such as 65% of the overall maximum amplitude 886.

[0191]If the current local maximum amplitude 887 is not greater than the
local maximum amplitude threshold, No in Block 906, no determination of a
maximum amplitude is made for that sub-segment, Block 908, and the device
determines whether the local maximum amplitude 887 and local amplitudes
888 have been determined for all of the four sub-segments 885, Block 922,
described below. If the current local maximum amplitude 887 is greater
than the local maximum amplitude threshold, Yes in Block 906, the device
determines whether the sub-segment 885 includes a local amplitude 888
that is greater than a local amplitude threshold, Block 918.

[0192]According to an embodiment of the present invention, the local
amplitude threshold of Block 918 is a percentage of the local maximum
amplitude 887 for that sub-segment 885, such as 70% of the local maximum
amplitude 887 for that sub-segment 885, for example. If the sub-segment
885 does not include a local amplitude 888 that is greater than the local
amplitude threshold, No in Block 918, a determination cannot be made for
that sub-segment 885, Block 920, and the device determines whether a
local maximum amplitude 887 and local amplitudes 888 have been determined
for all of the four sub-segments 885, Block 922, described below.

[0193]If the sub-segment 885 includes at least one local amplitude 888
that is greater than the local amplitude threshold, Yes in Block 918, or
if no determination of a local maximum amplitude was made, Block 908, the
device determines whether the local maximum amplitude 887 and local
amplitudes 888 have been determined for all of the four sub-segments 885,
Block 922. If all of the local maximum amplitude 887 and local amplitudes
888 have not been determined for all of the sub-segments 885, Yes in
Block 922, the process is repeated for the next sub-segment 885.

[0194]In order to obtain a corrected heart rate for a three second window,
the device must be successful in locating a local maximum amplitude 887
and at least one local amplitude 888 in a predetermined number of the
sub-segments, such as two sub-segments for example. Therefore, once the
determination of a local maximum amplitude 887 and the local amplitudes
888, Blocks 904-918, has been made for all of the sub-segments 885, No in
Block 922, a determination is made as to whether a local maximum
amplitude 887 and at least one local amplitude 888 were found to exist in
the predetermined number of sub-segments 885, Block 924. If a local
maximum amplitude 887 and at least one local amplitude 888 greater than
the local amplitude threshold were not found for the predetermined number
of sub-segments 885, No in Block 924, no corrected heart rate is
determined for the three second window, Block 914. If a local maximum
amplitude 887 and at least one local amplitude 888 greater than the local
amplitude threshold was found for the predetermined number of
sub-segments 885, Yes in Block 924, rates are determined for each of the
four sub-segments 885, Block 926, and the corrected rate is determined
using the determined sub-segment rates, Block 928.

[0195]If the maximum amplitude for the three second window is not greater
than the absolute value of the minimum amplitude for the three second
window, No in Block 901, the device uses the minimum amplitudes to
determine the corrected rates, Block A in FIG. 20A. In particular, as
illustrated in FIG. 20B, the device divides the three second window 934
into a predetermined number of window sub-segments 935, Block 940, and
determines an overall minimum amplitude 936 associated with the entire
window, Block 942. A local minimum amplitude 937 is determined for one of
the sub-segments 935, Block 944, and a determination is made as to
whether the local minimum 937 for that sub-segment 935 is less than a
predetermined local minimum amplitude threshold, Block 946. According to
an embodiment of the present invention, the predetermined local minimum
amplitude threshold is a percentage of the overall minimum amplitude 936
for the sensing window, such as 65% of the overall minimum amplitude 936.

[0196]If the current local minimum amplitude 937 is not less than the
local minimum amplitude threshold, No in Block 946, no determination of a
minimum amplitude is made for that sub-segment, Block 948, and the device
determines whether the local minimum amplitude 937 and local amplitudes
938 have been determined for all of the four sub-segments 935, Block 956,
described below. If the current local minimum amplitude 937 is less than
the local minimum threshold, Yes in Block 946, the device determines
whether the sub-segment 935 includes local amplitudes 938 that are less
than a local amplitude threshold, Block 952.

[0197]According to an embodiment of the present invention, the local
amplitude threshold of Block 952 is a percentage of the local minimum
amplitude 937 for that sub-segment 935, such as 70% of the local minimum
amplitude 937 for that sub-segment 935, for example. If the sub-segment
935 does not include at least one local amplitude 938 that is less than
the local amplitude threshold, No in Block 952, a determination cannot be
made for that sub-segment 935, Block 954, and the device determines
whether the local minimum amplitude 937 and local amplitudes 938 have
been determined for all of the four sub-segments 935, Block 956,
described below.

[0198]If the sub-segment 935 includes at least one local amplitude 938
that is less than the local amplitude threshold, Yes in Block 952, or if
no determination of a local minimum amplitude was made, Block 948, the
device determines whether the local minimum amplitude 937 and local
amplitudes 938 have been determined for all of the four sub-segments 935,
Block 956. If all of the local minimum amplitudes 937 and local
amplitudes 938 have not been determined for all of the sub-segments 935,
Yes in Block 956, the process is repeated for the next sub-segment 935.

[0199]In order to obtain a corrected heart rate for a three second window,
the device must be successful in locating a local minimum amplitude 937
and at least one local amplitude 938 in a predetermined number of the
sub-segments, such as two sub-segments for example. Therefore, once the
determination of a local minimum amplitude 937 and the local amplitudes
938, Blocks 942-952, has been made for all of the sub-segments 935, No in
Block 956, a determination is made as to whether a local minimum
amplitude 937 and at least one local amplitude 938 less than the local
amplitude threshold has been found for the predetermined number of
sub-segments 935, Block 958. If a local minimum amplitude 937 and at
least one local amplitude 938 less than the local amplitude threshold
were not found for the predetermined number of sub-segments 935, No in
Block 958, no corrected heart rate is determined for the three second
window, Block 960. If a local minimum amplitude 937 and at least one
local amplitude 938 less than the local amplitude threshold were found
for the predetermined number of sub-segments 935, Yes in Block 958, rates
are determined for each of the four sub-segments 935, Block 962, and the
corrected rate is determined using the determined sub-segment rates,
Block 964.

[0200]FIG. 21 is a flowchart of a method for determining a corrected rate
according to an embodiment of the present invention. The flowchart of
FIG. 21 describes the determination of the sub-segment rates when the
maximum amplitude is utilized, Yes in Block 901 of FIG. 20A. The
determination of the sub-segment rates when the minimum amplitudes are
utilized, No in Block 901, is similar to the determination when the
maximum amplitude is utilized, differing only in that minimum amplitudes
are used in place of the maximum amplitudes, and therefore is not
included merely for brevity sake.

[0201]In particular, according to an embodiment of the present invention,
in order to determine sub-segment rates, Block 926, the device determines
interval rates associated with the local maximum amplitude 887 and the
local amplitudes 888 within a sub-segment 885 that were determined in
Block 918, and a sub-segment rate for each of the sub-segments 885 is
determined based on the RR interval rates, Block 932. For example, as
illustrated in FIG. 19A, one of the sub-segments 885 includes the local
maximum amplitude 887 and two local amplitudes 888 so that two RR
intervals, RR1 and RR2 are determined as the intervals for that
sub-segment 885 in Block 930. The rate for that sub-segment 885 is
determined in Block 932 based on RR intervals RR1 and RR2, such as based
on the average of the RR intervals RR1 and RR2. If a sub-segment rate has
not been determined for all of the sub-segments 885 for which a local
maximum amplitude 887 and at least one local amplitude 888 were
previously determined, Yes in Block 334, the device determines a
sub-segment rate for the next sub-segment 885, Blocks 930 and 932.

[0202]For example, the next sub-segment 885 includes the local maximum
amplitude 887 and one local amplitude 888, so that the sub-segment rate
is equal to the RR interval RR3. The next sub-segment 885 includes the
local maximum amplitude 887 and one local amplitude 888, so that the
sub-segment rate is equal to the RR interval, RR4. No sub-segment rate is
determined for the last sub-segment 885 since, although the last
sub-segment included a local maximum amplitude, which corresponded to the
overall maximum amplitude for the window 886, the last sub-segment did
not include at least one additional local amplitude 888 that was
determined in Block 918 to be greater than the local amplitude threshold.

[0203]Once a rate has been determined for each of the qualified
sub-segments 885, No in Block 934, the device determines the corrected
rate for the three second window, Block 936, based on the determined
sub-segment rates. For example, the device sorts the determined
sub-segment rates from fastest to slowest, and uses the average of a
predetermined number of the sorted sub-segment rates to determine the
corrected rate. In particular, according to an embodiment of the present
invention, the device makes the determination based on the number of
sub-segment rates that were determined, so that if a sub-segment rate was
determined for all four of the sub-segments, the corrected rate is
determined as the average of the second and third sub-segment rates of
the sorted sub-segment rates. If a sub-segment rate was determined for
three of the four sub-segments, the corrected rate is determined as the
average of the second and third sub-segment rates of the sorted
sub-segment rates, and if a sub-segment rate was determined for only two
of the sub-segments, the corrected rate is determined as the average of
the first and second sub-segment rates of the sorted sub-segment rates.

[0204]FIG. 22 is a flowchart of a method for determining a corrected rate
according to an embodiment of the present invention. FIG. 23 is an
exemplary schematic diagram of a buffer of RR intervals generated
according to an embodiment of the present invention. In particular, FIG.
23 illustrates an example of a buffer of twelve RR intervals 996 that
were stored during an instance of oversensing due to a slow monomorphic
ventricular tachycardia with a wide QRS complex, in which the first
interval is a long interval L, the second interval is a short interval S,
the third interval is a long interval L, and so forth.

[0205]According to another embodiment of the present invention, in order
to determine the corrected heart rate in response to the determination of
oversensing, the device identifies short-long interval patterns by
looking sequentially at each RR interval, Block 970, starting with the
first RR interval of the stored buffer of 12 RR intervals, and generating
an update RR interval to be stored in an updated RR interval buffer based
on a determination as to whether the interval is a short interval or a
long interval. In particular, as illustrated in FIG. 22, in order to
identify short-long interval patterns, the device obtains an RR interval,
Block 970, and determines whether the RR interval is less than or equal
to an interval threshold, Block 972. If the current RR interval is not
less than the interval threshold, No in Block 972, the RR interval is
likely a long interval and therefore not a first interval of a short-long
interval pattern. Therefore the current RR interval is set as the next RR
interval of the updated RR intervals, Block 974. If the current RR
interval is less than or equal to the interval threshold, Yes in Block
972, the current RR interval is likely a short interval, and therefore
the device obtains the next RR interval immediately subsequent to the
current RR interval, Block 976, and sets the next updated RR interval
equal to the sum of the current RR interval, i.e., the short RR interval
and the next RR interval, Block 978.

[0206]According to an embodiment of the present invention, the interval
threshold is the sum of the blanking period, i.e., 180 ms for example,
and a rate correction constant associated with a maximum width of the
particular waveform that the device is attempting to identify. For
example, in an embodiment of the present invention, the rate correction
constant is set as 20 ms when the device is attempting to identify
instances of oversensing due to a slow monomorphic ventricular
tachycardia with a wide QRS complex.

[0207]Once an updated RR interval has been determined based on a single
long RR interval, Block 974, or based on the sum of a short interval and
a next interval, Block 978, the device then determines in Block 980
whether each one of the 12 buffered RR intervals have been either
identified as being a long interval, Block 972, and therefore used alone
to generate a next update interval, or utilized in combination with a
preceding interval determined to be a short interval to generate the next
updated interval Blocks 976-978. If all of the 12 buffered RR intervals
have not been utilized to generate a next updated RR interval, Yes in
Block 980, the process is repeated with the next available buffered RR
interval, Block 970. If all of the 12 buffered RR intervals have been
utilized to generate an updated RR interval, No in Block 980, the device
sets the corrected rate based on the buffer of updated RR intervals,
Block 982. For example, according to an embodiment of the present
invention, the device sets the corrected rate equal to the mean of the
second through the last updated RR interval of the buffer of updated RR
intervals.

[0208]As illustrated in FIGS. 22 and 23, since the first interval of
buffer 996 is a long interval, No in Block 972, the first updated
interval of a buffer of updated RR intervals 998 is set equal to the
first interval of buffer 996. Since the second interval of buffer 996 is
a short interval, the second updated interval of buffer 998 is set equal
to the sum of the second and third intervals of buffer 996. The process
is repeated, resulting in the buffer of updated RR intervals 998
containing seven updated intervals, so that the corrected rate is
determined in Block 982 as the mean of the second through the seventh
updated RR interval from buffer 998.

[0209]According to an embodiment of the present invention, the interval
threshold in Block 972 may correspond to a trimmed mean of the buffered
12 RR intervals, such as a trimmed mean of the third through the tenth
sorted RR intervals, as utilized above for example TM. In this way, in
the determination of the corrected rate, the device first sorts the 12
buffered RR intervals from smallest to largest, and sets the interval
threshold utilized in Block 972 as the trimmed mean of the third through
tenth RR interval. Once the interval threshold has been set, the rate
correction process, Blocks 970-982, is performed as described above. As a
result, the corrected rate is determined in response to both a sorted and
a non-sorted buffer of the buffer of 12 RR intervals.

[0210]FIG. 24 is a flowchart of a method for determining a corrected rate
according to an embodiment of the present invention. FIG. 25 is an
exemplary schematic diagram of a buffer of RR intervals generated
according to an embodiment of the present invention. In particular, FIG.
25 illustrates an example of a buffer of twelve RR intervals 997 that
were stored during an instance of oversensing due to a slow monomorphic
ventricular tachycardia with a wide QRS complex, in which the first
interval is a long interval L, the second interval is a short interval S,
the third interval is a long interval L, the fourth and fifth intervals
are short intervals, and so forth. In order to further insure that rate
correction is only applied when a short-long RR interval pattern exists,
the device performs the rate correction according to another embodiment
of the present by looking sequentially at two adjacent RR intervals,
Block 984, starting with the first two RR intervals of the stored buffer
of 12 RR intervals, generates an updated RR interval buffer based on
whether the first RR interval of the adjacent RR intervals is a short
interval and the second RR interval is a long interval, and determines
the corrected rate based on the updated intervals in the updated RR
interval buffer. In particular, as illustrated in FIG. 24, the device
obtains adjacent RR intervals, Block 984, starting with the first and
second RR interval for example, and determines whether the first RR
interval of the two adjacent RR intervals is less than or equal to an
interval threshold, Block 986. If the first RR interval is not less than
or equal to the interval threshold, No in Block 986, the first RR
interval is likely a long interval, negating the possibility that the
current two intervals correspond to a short-long interval sequence, and
therefore the first RR interval is set as the next updated RR interval of
the buffer of updated RR intervals, Block 988.

[0211]If the first RR interval is less than or equal to the interval
threshold, Yes in Block 986, the current RR interval is likely a short
interval, and therefore the device determines whether the second RR
interval, i.e., the subsequent interval adjacent to the first RR
interval, is greater than the interval threshold, Block 990. If the
second RR interval is not greater than the interval threshold, No in
Block 990, both the first RR interval and the second RR interval are
likely short intervals, negating the possibility that the current two RR
intervals correspond to a short-long interval sequence, and therefore the
first RR interval is set as the next updated RR interval of the buffer of
updated RR intervals, Block 988. If the first RR interval is less than or
equal to the interval threshold, Yes in Block 986, and the second RR
interval is greater than the interval threshold, Yes in Block 990,
indicating that the first RR interval is a short interval and the second
RR interval is a long interval, the device sets the next updated RR
interval in the stored buffer of RR intervals based on the two current
intervals, Block 992. For example, according an embodiment of the present
invention, the device sets the next updated RR interval equal to the sum
of the current first RR interval and second RR interval.

[0212]According to an embodiment of the present invention, the interval
threshold of Blocks 986 and 990 is the sum of the blanking period, i.e.,
180 ms for example, and a rate correction constant associated with a
maximum width of the particular waveform that the device is attempting to
identify. For example, in an embodiment of the present invention, the
rate correction constant is set as 60 ms when the device is attempting to
identify instances of oversensing due to a slow monomorphic ventricular
tachycardia with a wide QRS complex.

[0213]Once an updated RR interval has been determined based on a single
long RR interval, Block 988, or based on the sum of a short interval and
a long interval, Block 992, the device then determines in Block 994
whether each one of the 12 buffered RR intervals have been either
identified as being a long interval, Block 986, and therefore used alone
to generate a next update interval, Block 988, or utilized in combination
with an adjacent RR interval determined to form a short-long interval
sequence, Block 992. If all of the 12 buffered RR intervals have not been
utilized to generate a next updated RR interval, Yes in Block 994, the
process is repeated with the next available adjacent buffered RR
intervals, Block 984. If all of the 12 buffered RR intervals have been
utilized to generate a next updated RR interval, No in Block 994, the
device sets the corrected rate based on the updated RR intervals, Block
996. For example, according to an embodiment of the present invention,
the device sets the corrected rate equal to the mean of the second
through the last updated RR intervals of the updated RR intervals.

[0214]As illustrated in FIGS. 24 and 25, since the first interval of
buffer 997 is a long interval, No in Block 986, the first updated
interval of a buffer of updated RR intervals 999 is set equal to the
first interval of buffer 997. Since the second interval of buffer 997 is
a short interval, Yes in Block 986, and the third interval is a long
interval, Yes in Block 990, the second updated interval of buffer 999 is
set equal to the sum of the second and third RR intervals of buffer 997.
Since the fourth RR interval of buffer 997 is a short interval, Yes in
Block 986, and the fifth RR interval is a short interval, No in Block
990, the third updated interval of buffer 999 is set equal to the fourth
interval of buffer 997. Since the fifth interval was not included in
generating the third updated interval, the device utilizes the fifth and
sixth intervals of buffer 997 in the next iteration, resulting in a
determination that the fifth RR interval of buffer 997 is a short
interval, Yes in Block 986, and the sixth RR interval is a long interval,
Yes in Block 990, so that the fourth updated RR interval of buffer 999 is
set equal to the sum of the fifth and sixth intervals of buffer 997. The
process is repeated, resulting in the buffer of updated RR intervals 998
containing eight updated intervals, so that the corrected rate is
determined in Block 996 as the mean of the second through the seventh
updated RR interval from buffer 999.

[0215]The present invention may utilize other embodiments for determining
the corrected rate in place of the embodiments described above. For
example, according to another embodiment of the present invention, in
order to determine the corrected heart rate in response to oversensing,
the device sorts the 12 buffered RR intervals from smallest to largest so
that the corrected rate is determined based on predetermined RR intervals
from the sorted RR intervals. For example, according to an embodiment of
the present invention, the corrected rate is determined as the sum of the
mean of second through fifth smallest RR interval and the mean of the
eighth through eleventh RR interval.

[0216]According to another embodiment of the present invention, in order
to determine the corrected heart rate in response to oversensing, the
device sorts the 12 buffered RR intervals from smallest to largest so
that the corrected rate is determined based on one or more predetermined
RR intervals from the sorted RR intervals and the spectral width of the
signal for the three-second segment associated with each channel ECG1 and
ECG2, determined as described above. For example, according to an
embodiment of the present invention, the corrected rate is determined as
the sum of the 9th fastest RR interval of the sorted RR intervals
and the spectral width associated with the three second segment for the
channel ECG1 or ECG2.

[0217]FIG. 26 is a flowchart of a method for detecting cardiac events in a
medical device according to an embodiment of the present invention. The
method described in FIG. 26 is similar to the method described above in
reference to FIG. 15, and includes the determination of whether
oversensing has occurred in a situation where only one of the sensing
channels ECG1 and ECG2 has been determined to be reliable, described
above in reference to FIG. 16B. Therefore, a description of those steps
already described above in reference to FIGS. 15 and 16B will not be
repeated for brevity sake. The embodiment of FIG. 26 differs only in that
it includes additional steps associated with a timer for controlling the
rate at which the oversensing determination is performed, which results
in a reduction in the amount of processing that occurs during the
oversensing determination, thereby increasing the battery life of the
device.

[0218]In particular, as illustrated in FIG. 26, once the device has
determined the final heart rate estimate for each channel ECG1 and ECG2
using the methods described above in reference to Blocks 322-334 of FIG.
7A, for example, a determination is made as whether a timer has expired,
Block 861. The timer of Block 861, which is initialized as being expired,
i.e., equal to zero, is set during the oversensing determination as will
be described below. According to an embodiment of the present invention,
the timer is set as 750 ms, which corresponds to the time associated with
a single sub-segment of the three-second window, although any desired
time period for controlling the rate at which the oversensing
determination is performed may be utilized.

[0219]If the timer has not expired, i.e., at least one of the oversensing
criteria has been satisfied within the last 750 ms, No in Block 861, the
determination of the heart rate estimate for each channel is repeated,
Blocks 322-334. If the timer has expired, i.e., at least one of the
oversensing criteria has not been satisfied within the last 750 ms, Yes
in Block 861, the determination is then made as to whether the final
heart rate estimates is greater than the predetermined VT/VF threshold,
Block 336, described above. If the final heart rate estimates are not
greater than the VT/VF threshold, a new heart rate estimate is
determined, Blocks 322-334. If the final heart rate estimates are greater
than the VT/VF threshold, the oversensing determination is made for the
current generated heart rate estimate. According to the embodiment of the
present invention illustrated in FIG. 26, the oversensing determination
includes all three of the previously described oversensing
characteristics, spectral width, the metric of signal energy content, and
the heart rate metric difference, with the heart rate metric difference
being the initial oversensing characteristic. However, as mentioned
above, embodiments of the present invention may include any number or
combination of the oversensing characteristics may be utilized, performed
in any desired sequence.

[0220]In particular, for example, if the first channel ECG1 was determined
to be unreliable and the second channel ECG2 was determined to be the
reliable channel, a heart rate metric difference is determined for the
reliable channel ECG2, Block 858, and a determination as to whether the
heart rate metric difference is less than or equal to the heart rate
metric difference threshold, Block 860. If the heart rate metric
difference is greater than the heart rate metric difference threshold, No
in Block 860, oversensing is determined to not be occurring, and the
device transitions from the not concerned state 302 to the concerned
state 304. If the heart rate metric difference is less than or equal to
the heart rate metric difference threshold, Yes in Block 860, the timer
is set, Block 867. A spectral width is then determined for the reliable
channel ECG2, Block 850, and a determination is made as to whether the
spectral width is less than or equal to the spectral width threshold,
Block 852.

[0221]If the spectral width is not less than or equal to the spectral
width threshold, No in Block 852, oversensing is determined not to be
occurring, and the device transitions to the concerned state, Block 304.
If the spectral width is less than or equal to the spectral width
threshold, Yes in Block 852, a metric of signal energy content is
determined for the reliable channel ECG2, Block 854, and a determination
is made as to whether the metric of signal energy content is greater than
the metric of signal energy content threshold, Block 856.

[0222]If the metric of signal energy content is not greater than the
metric of signal energy content threshold, No in Block 856, oversensing
is determined to not be occurring, and the device transitions from the
not concerned state 302 to the concerned state 304. If the metric of
signal energy content is greater than the metric of signal energy content
threshold, Yes in Block 856, oversensing is determined to likely be
occurring and the corrected rate is determined, Block 822 of FIG. 15,
using the rate correction technique described above. If the corrected
rate is greater than the predetermined VT/VF threshold, Yes in Block 824,
oversensing is determined to not be occurring, and the device transitions
from the not concerned state 302 to the concerned state 304.

[0223]If the corrected heart rate is not greater than the predetermined
VT/VF threshold, No in Block 824, a new current heart rate estimate is
then determined, Blocks 322-332 and the process is repeated. However,
since no oversensing was determined to be occurring for the previous
heart rate estimate, and therefore the timer was initiated in Block 867,
once the next heart rate estimate has been determined, Block 334, the
determination of oversensing will be delayed until a desired amount of
new sensing data has been analyzed, since the timer will not be expired,
No in Block 861. While the amount of time necessary for the oversensing
determination and the rate correction to occur will vary depending on the
number of oversensing characteristics utilized, the inventor has found
that the oversensing determination process of FIG. 26 may take
approximately 50 ms to be completed. By setting the timer in Block 867 as
being 750 ms, for example, the amount of time between sequential
oversensing determinations is increased so that a more appropriate amount
of sensing data is reviewed before the oversensing determination is
repeated. In this way, the amount of processing resulting during the
oversensing determination is reduced, increasing the battery life of the
device.

[0224]While the determination of oversensing and the generation of the
corrected rate according to the present invention has been described
using two sensing vectors, it is understood that the present invention
could also be utilized to determine the presence of oversensing and to
generate the corrected rate using only one sensing vector. Similarly,
while the present invention is described when utilized in a subcutaneous
device, it is understood that the oversensing detection and subsequent
rate correction of the present invention may also be utilized in known
transvenous systems, such as the transvenous system described in U.S.
Pat. No. 7,133,718 to Bakken et al., for example, incorporated herein in
it's entirety.

[0225]It will be apparent from the foregoing that while particular
embodiments of the invention have been illustrated and described, various
modifications can be made without departing from the spirit and scope of
the invention. Accordingly, it is not intended that the invention be
limited, except as by the appended claims. For example, as illustrated in
FIG. 71, during the noise determination of Block 744, the determination
is made for each channel ECG1 and ECG2 as to whether the channel is
corrupted by noise as described above. However, according to an
embodiment of the present invention, once noise is determined to be
present in either channel, No in Blocks 380, 382 or 388, Yes in Block 384
of FIG. 7C, both channels are classified as being not shockable, Block
748.

[0226]If noise is not present in either channel ECG1 and ECG2, No in Block
744, a determination is made as for each channel ECG1 and ECG2 as to
whether the channel is in a VF shock zone. For example, according to an
embodiment of the present invention, a determination is made that channel
ECG1 is in the VF shock zone, Yes in Block 748, if, for channel ECG1,
both the low slope content is less than the first boundary 502 and the
spectral width is less than the second boundary 504, as described above.
The three second segment for that channel ECG1 is then determined to be
shockable, Block 750 and the associated buffer for that channel is
updated accordingly. If either the low slope content for the channel is
not less than the first boundary 502 or the spectral width is not less
than the second boundary, the channel ECG1 is determined not to be in the
VF shock zone, No in Block 748, the three second segment for that channel
ECG1 is then determined to be not shockable, Block 752, and the
associated buffer is updated accordingly.

[0227]Similarly, a determination is made that channel ECG2 is in the VF
shock zone, Yes in Block 754, if, for channel ECG2, both the low slope
content is less than the first boundary 502 and the spectral width is
less than the second boundary 504, as described above. The three second
segment for that channel ECG2 is then determined to be shockable, Block
756 and the associated buffer for that channel is updated accordingly. If
either the low slope content for the channel is not less than the first
boundary 502 or the spectral width is not less than the second boundary,
the channel ECG2 is determined not to be in the VF shock zone, No in
Block 754, the three second segment for that channel ECG2 is then
determined to be not shockable, Block 758, and the associated buffer is
updated accordingly.

[0228]Once the classification of both of the channels ECG1 and ECG2 as
being either shockable, Block 752 and 758, or not shockable, Blocks 748,
754 and 760, a determination is made as to whether the device should
transition from the concerned state 304 to the armed state 306, Block
762. The determination of whether the device should transition from the
concerned state 304 to the armed state 306 in Block 762, in addition to
the subsequent determination of whether to transition from the concerned
state 304 to the not concerned state 302 in Block 764 are similar to the
determination of whether the device should transition from the concerned
state 304 to the armed state 306 in Block 370, and to the determination
of whether to transition from the concerned state 304 to the not
concerned state 302 in Block 372 in FIG. 7B described above, and
therefore will not be repeated for the sake of brevity.

[0229]Some of the techniques described above may be embodied as a
computer-readable medium comprising instructions for a programmable
processor such as microprocessor 142, pacer/device timing circuit 178 or
control circuit 144 shown in FIG. 3. The programmable processor may
include one or more individual processors, which may act independently or
in concert. A "computer-readable medium" includes but is not limited to
any type of computer memory such as floppy disks, conventional hard
disks, CR-ROMS, Flash ROMS, nonvolatile ROMS, RAM and a magnetic or
optical storage medium. The medium may include instructions for causing a
processor to perform any of the features described above for initiating a
session of the escape rate variation according to the present invention.

[0230]While a particular embodiment of the present invention has been
shown and described, modifications may be made. It is therefore intended
in the appended claims to cover all such changes and modifications, which
fall within the true spirit and scope of the invention.

Patent applications by Karen J. Kleckner, New Brighton, MN US

Patent applications by Paul G. Krause, Shoreview, MN US

Patent applications by Raja N. Ghanem, Edina, MN US

Patent applications by Robert W. Stadler, Shoreview, MN US

Patent applications by Xusheng Zhang, Shoreview, MN US

Patent applications in class Parameter control in response to sensed physiological load on heart

Patent applications in all subclasses Parameter control in response to sensed physiological load on heart